There's not much activity over here so I'll chime in. Disclaimer first. I don't have either style of printer but I think both look to be fine printers. The x rods on the PM3 makes it a whole lot easier to add a second extruder than the side by side ones on the mendel90. There are more designs out there for a dual extruder carriage for the PM3. I have a Prusa2 which has a similar x axis setup to the mendel 90 and have spent a great deal of time looking for a good 2 extruder carriage. I never found one so I built another printer with the PM3 style x axis
On this new printer I set up dual j head's then switched over to dual e3d v6 and a few months ago swapped those out to a Chimera. Now I'm wishing I had went with a Cyclops. Although that diamond hot end looks intriguing. To answer your question, I'm guessing the Prusa I3 would be a better choice.
Ray
Edited 2 time(s). Last edit at 10/20/2016 08:15PM by Shank man.