Here's a few pics w/ my winter setup. 18" TSW Nogaros - first of all I prefer the look of these to my stock sport 19"s (I like the lip and open area between spokes so you can see the brakes), secondly they're considerably lighter, probably nearly 10lbs per wheel, lastly they ride better.
I can corner better and have better traction w/ the 19"s, but then again alot of that has to do w/ the rubber mated to it, I'm sure if I had summer-only high performance on 18"s it'd be pretty good too.
I think light weight 19"s are probably the best compromise between comofort/looks/performance, but I'm not going to sit here and tell you 19"s or 20"s ride any where close to as nice comfort-wise as an 18" rim would (they won't).
I think 18" is really perfect for DD duties
'08 GT sport/premium, tints 20/35, 18" TSW nogaros w/ bridgestone blizzak winter, 19" sport oem w/ hankook evo summer, Philips crystalvision bulbs, debadged, Vararam OTR, JBA 1-3/4" LT's, ported TB, MF axlebacks, MF X-pipe, DBA-powerslot brakes, russel SS lines, hawk ceramic pads, GXP shocks/struts, lovells poly strut mounts, Slowhawk Tuned 344/356
Gone -'04 GTO A4 2.5" custom exhaust, sweet thunder muffs, H pipe, Z06 heads, Baer brake it, 20% tints, torrid red, AEM brute force intake, tuned