If you do ultras you might want one with a longer battery life. Some of the Garmins say 8 hours, which tends to mean 6 in practice. Not long enough for some ultra distances. The 305 is still very popular with ultra runners even though it's a much older model, due to its 12 hour battery life. The 310 and 910 have 20 hour batteries but cost a lot more.
Apart from the battery life thing, I think I'd be happy with just about any Garmin. Mine is a 405 and I don't use even half the features available on it. I just like knowing distance and average pace, and being able to log all my training online.
There are other makes of GPS watch too. I think Suunto watches are quite good.
