Go for the 500 (I'm biased, though).
Seriously, you won't be as comfortable on the highway on the 250, and you might get bored with the 250 quickly. The 500 has a little bit more to offer.
Definitely buy a used bike. Lots of people start out on EX500s--you should be able to find a 99-01 with low miles, maybe a bit dinged up, but for thousands less than new. I got my '99 last year for $3K with a new rear tire (original had a nail in it) and 18 months left on the extended warranty that the first guy bought. It's scratched up, but that's one less thing I had to worry about (putting the first scratch on a new bike)!