The 300 solves the few things that could be considered wrong with the 200, it has a removable power cord (which is longer than the non-removable 200 cord), and an auto-shut off feature. Also I would say it is a bit nicer aesthetically, but that's more of matter of taste.
Owning the 200 the only genuinely annoying thing is the ~3 foot power cord, keep meaning to get an extension cord but still haven't gotten around to it. Not sure if either of the improvements are worth an extra $80 over the 200 though.
Also you could save more money by buying a flat coil on amazon or dhgate as you can get them for $30-35 compared to the $60 from Auber. Just make sure it's 100 watts and has the standard pin configuration.
The domeless ceramic enail is supposed to be great, but some people found it didn't fit their flat coil so they had to unwind the coil a bit with a screwdriver. This was a while ago so domeless might have changed the design, but just something to be aware of. Also not sure if you were including a carb cap, but you'll definitely want one of those as well.
You might also want to consider a quartz/titanium hybrid nail which has a quartz dish but the rest of the parts are titanium. You can get an all american version from New Vape, or get a Chinese version from DHGate. I have a slightly different model of the Chinese version and it tastes pretty great. Otherwise you might want to check out D-Nail or Highly Educated if you have more money. If you want an all quartz nail you are basically going to have to ditch the flat coil and get either a 16mm or 20mm barrel coil. If you decide on the barrel coil you can get a pretty good quartz e-nail from hymanquartz01 on DHGate, or if you want to spend more money you could get a Liger Banger with quartz or sic insert.
http://www.dhgate.com/store/product/2016-6-in-1-dualiti-enail-ti-qtz-enail-titanium/373183422.html
http://www.newvape.com/hybrid-quartz-enail-14-18mm-female-9053/
One final thing to note about ordering from DHGate right now is that it's Chinese New Year so your order might not ship for a week or 2.