I voted the 250 and go up if needed however I don't plan to use it much. I will only sign on for one month to see if there is any noticeable difference using the built in 3G VS. Using my iPhone to tether.
If I was going to use the 3G long term I would go with unlimited for sure. According to my ATT app with 28 of my 30 day cyle used I am at 2914.4 MB used