I'd get a
prusa i3 over the simple metal. Both use open source firmwares (modified versions of marlin) so you can use any 3rd party software that uses standard reprap gcode. While the i3 doesn't come with dual extruders, it is much easier to add a second one than it would be to the simple metal as there are already plenty of designs out there to do so. also, the hotend that comes with the i3 is much better than the ubis that comes with the simple metal, especially if you want to print fast (>50mm/s or so) or with high temp materials. if you decide to get the i3, beware of the cheap kits. the one I linked, which is what I would recommend getting, is from the original designer and uses good parts. most of the cheap i3 kits have fragile acrylic frames, knockoff e3d hotends, etc.