Although the 21's look really cool, in the long run many will tell you that 19's are more practical. They are quieter, have a smoother ride, last longer (tires), cost less (tires), and are less subject to damage (both wheels and tires). I've heard that many people purchase 21's but end up switching to 19's due to the above. Also (and this was a deal breaker for me) it's difficult to find anything except summer performance radials for the 21's (no all season radials which is what I always run). I believe you cannot put chains on the 21's either, if that's ever anything you'd do.
The 21's probably handle a bit better in corners, but comparing that singular gain to the multitude of detractions, well you'll need to make your own mind up. Of course you could always go the optimum route and that is have both 21's (for summer) and 18's (for winter). But if roads where you live have pot hole problems, you should probably skip the 21's altogether.
I notice you're in Sacramento, do you ski? If so, don't plan on taking the 21's.