having owned a poly800 and played the microkorg (the first one) i would definitely say the 800 is more unique sounding. as stab said its good for running through effects (phaser! i wish i had my Murf at the same, that would probably sound pretty incredible) decent analog chorus. it does bass ok but its best at strings, organ, and pads i think. also check out the HAWK-800 mod/upgrade/hack gives it ton more sound and modulation possibilities, i sold mine off before i could get that done so i could buy a drum machine which i then sold to buy a Murf ironically.
microkorg is neat its got a vocoder and built in effects, arppegiator so its a more versatile. and you can probably dial in "vintage" sounds a lot easier.
so i guess it depends on what your looking to use it for
My modular so far: Q104, Q106, Q107, Q108, Q109 x2 , Q116, Q118, Q127 w/Q140, Q130, STG Wave Folder, Encore UEG playing with Moog Voyager, VX-351, CP-251, MF-104M (x2 STEREO!) and Yamaha RX5