I am just a newb, but I think the benefits of a top mounted spool system outweigh any others. it has the least travel time to the extruder, the least ability to get tangled, it takes up vertical space only which anyone has, and you can easily feed it through a filter. that being said, I attempted designing my own spool holder specific to the Prusa i3 x - but I found a great design (not including a filter) on thingiverse, which I will probably test out. http://www.thingiverse.com/thing:871223

I do have a number of other spool mounts saved in one of my collections on thingiverse. here are a few of my favorite universal spool mounts.
http://www.thingiverse.com/thing:998774
http://www.thingiverse.com/thing:296971