After many hours of reading and watching videos, I have come to some conclusions:
(that seems to me to be right, but any objection will help)


1. Using Dual Head for dual filament printing, seems to be more "professional" than "many to one" head option.
It needs much less filament, can have different temperature for various types of filament, but it has 3 disadvantages:
A. Setting the same height for both heads can become a nightmare.
B. Dripping of non active head can destroy a whole printing,
specially if one of the filaments is water resolved.
C. Useful printing area is reduced in the axis that heads are positioned side by side.


So Dual Head option is rejected, and a 2 (or 3) to 1 head will be used.
Of course the "separate" mixing systems that "assemble" many filaments to one,
are an other option that can be used without any modification to a single filament printer.


2. The most "straightforward" system of printing seems to be (to mine understanding) the Delta system.
Plate is not moving at all, there are not any tooth belts, just 3 step motors to turn screw rods.
The only moving part is head and its holding rods, meaning high speed movement creates minimum shake.


The major drawback is their height, that it must be about double of the "useful" height.
And then the circular bed that must be big enough to be "equal" to a square bed printer.
Tevo Little Monster @ 720$ that can print on a 340mm diameter circle (about 240 X 240 square) and 500mm in height
And the machine height is about 1.2 meters ...
So this solution is out of my requirements, as I need a bad about 350X350 mm.


3. The 2nd solution that bed moving is not big issue is the one bed moves vertically.
Since this is done only on every layer change, construction shake is also minimal at high speeds.
The problem on the solutions provided, seems to the use of CoreXY system.
Very long tooth belts are used (equal to about the XY perimeter of the printer),
and the tension as also the proper alignment of the bar in which head is moving, are critical.


Bed vertical movement, is done by two screw bars at right and left of the bed,
an two pairs of cylindrical bars for guidance. It seems to have no problems.


4. Finally we have the -most used- moving bed solutions.
In those printing speed can't be high, because bed has a bigger mass than other solutions in their moving parts (plus the part itself),
in order to keep vibrations to a minimum.
But forgetting speed, if we have a stiff horizontal frame and another one for XZ plane, vibrations may be not an issue.


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So these are my conclusions about plus and minus of the 3 most used systems.
Selecting between CoreXY and Prusa ID3 types can't be a "clear" decision, without focusing on specific model details.
So I tried a comparison between these types, between two of the same company, TronXY
The X3SA of the Prusa ID3 moving bed type and the X5SA Pro of the CoreXY type.


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The moving bed solution is simple to assembly, as the moving bed is been assembled to main frame,
and the XZ plate is also assembled in a single piece.
So the final assembly is easy and there is not any need for trimming anything.
https://www.tronxy.com/x-series/tron...d-printer.html
Movement of the bed on Y axis is done on two thick guides that have adequate distance between them,
in order to assure strict movement on Y axis with out any jolt on X axis.
This is ensured by 3 rollers for each guide.
They have a significant distance between them (in each guide), in order to minimize the possibility of any movement on X, .
Movement is applied by a belt, and its tension can be regulated by a nut to the proper(?) elastic state.


Z axis movement is done on two vertical guides that are fixed by small plates to the main frame
and on top they have a connecting bar to keep them parallel between them.
Movement is applied by two screw rods, which are controlled by two motors, one for each rod.
The X axis bar is sliding to vertical bars by 3 rollers on each bar,
that have eccentric bases in order to permit fine tuning to ensure strict vertical movement.


Finally head is moving in X axis bar by tooth belt, and is held in bar by 3 rollers.
So movement is controlled by belts in X Y axis and screw rods in Z.
Rigidity of movement is depended on 5 sets of 3 rollers each.
Y axis movement is rigid enough, since roller distance on each set is a lot bigger than this of other axis.
This arrangement requires big space for the machine on Y axis, if someone has space restrictions.
Z axis movement is also rigid due to the screw rods,
and the rollers for the X axis are enough since head is a small part.


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The CoreXY solution is provided in totally DIY state.
https://www.tronxy.com/x-series/x5sa-pro.html
So there is a need for high skill on assembly procedures, as those of tuning.
Movement on 3 axis isa also done by belts in XY and screw rods on Z.


Bed is moving on Z axis, and its stability on X axis is been assured by two screw rods,
at right and left of X axis sides of bed, at the center distance of Y axis.
Y axis stability of bed is assured by 2 pairs of cylindrical rods and the bearings attached to bed frame.
It seems to have no problem, but since the two screw rods are not "connected" between them,
removing printed objects can move the bed frame and destroy leveling at X axis.
So every time a new print will start, leveling procedure must be repeated.
Fortunately this model provides an automatic leveling, so if the screw rods' positioning has been destroyed,
the system will bring back the bed leveling.


XY stability is depended on belts' tension, pulleys' stability, and rollers' proper positioning.
Belts' tension has to be tuned by the user on assembly, and there are not any nuts regulating tension.
So it is advised by some users to purchase fiber reinforced belts to keep their tension for longer period.
Also the rollers that are keep X bar parallel to X axis may be need some tuning (on the eccentric bases), in order to provide XY stability.
And the pulleys that keep belts in place, can have a little vertical movement that can create XY positioning problems.
A user that tested an X5SA, had terrible complaints for the print result, and it seems that he did not try to fix the printer.
https://www.youtube.com/watch?v=KiJg7f9ib4M
But others who also had same problems, has fixed them getting very good results.
https://www.youtube.com/watch?v=Pq3qjdaRnuY
https://www.youtube.com/watch?v=UN-2KeuzAKs
And one did some upgrades:
https://www.youtube.com/watch?v=RIz5-1uCqLg


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So my final conclusion is that a careful assembly with some upgrades on a CoreXY models can give very good results.
So the question is if am willing to proceed to improvements of to select a "ready to operate" solution.
The 400X400mm model of X5SA is a challenge, as it offers much bigger area for a good price ($550 vs $380 for 330X330 model)
To get a 400X400 ID3 model, Creality CR10-S4 400 is offered for $600, a very close price to Tronxy X5SA.

So I will continue searching reviews to find more "secrets" ...

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Very interesting is the capability to build huge moving bed solutions using custom design like this one.
https://www.youtube.com/watch?v=albFKmWW4nA
I believe that CoreXY solutions can also be created.