I would lean more towards the 500s. The ex250 is a great bike but it does have a few faults. You can slip the back tire when downshiting into to low a gear, and they can be pushed around on the highway on a windy day.
I sliped on the highway real quick on a windy day, when I noticed my bike being pushed sideways as I was going down the highway. Not the greatest feeling to have.
Also you will get tired of the 250s power within the first month, but that doesn't mean they are still fun to ride. They are fun little bikes for putting around town.
The best thing they have going for them are they are cheap, fun, and extreamly confidence inspiring motorcycles.