They are pretty much the same. The bed isn't wood anymore, and they just announced that all models will be getting the aluminum extruder that debuted on the SD4, so the acrylic jigsaw puzzle is finally gone. They haven't done anything to the X,Y and Z axes. I haven't seen many complaints or examples of Z banding lately, so maybe they improved quality control even though the design didn't change.
You should add glass plates and E3Ds to all of them, but I think that would be good advice for any printer in the $500 range. One change that is a bit of a problem is the move from Sanguinololu to Printrboard. The stepper drivers are soldered in to the Printrboard, so you can't replace the Extruder driver with an 8825 to get 1/32 microstepping. If you want to get rid of the moire pattern on the surface of the prints you need to increase E steps, and the only way to do that on the current Solidoodle is replacing the E stepper with a motor that does 400 steps per revolution.
If you are printing PLA, you might consider going a little more expensive and look into a Mendel90 or Ordbot kit at around $800. Making a printer farm from kits is more work, but you will get better quality at higher speeds. I run my Ord at 150mm/s and get better quality prints than I ever did with the Solidoodle with no trouble. It could go even faster, but requires more optimized temp settings to keep extrusion consistent at high speeds. The Mendel90 is developed and sold by Nophead, who is probably the most knowledgeable Reprap guy out there. It includes a filament dust wiper and nozzle fan, which I've only ever seen as a user-added feature.
