Items to note on two of those printers
Davinci you need to plan on printing replacement x and y bearing mounts. Voltivo forums is full of users whose machine has fallen apart with about 40 hours of use.
Solidoodle you will need to print a second print head if it has the older acrylic head. Solidoodle knows there is a problem and has never bothered to get rid of it. Also getting it shipped usually takes 1-2 months. Unless the place that you buy it from has them in stock, you are in for a wait. Solidoodle might have something larger in a version 3.

PrintRbot Metal Simple is a 6x6x6 volume and you can modify it to 8x8x10.

Good luck with what ever you go with.