How much data you need depends upon how you intend to use your devce. I have an iPhone. With that I almost never stream audio or video content. I only do emails, calendars, and a few web based apps, and I average around 400-50MB/month. I also use wfi when available, but I don't force myself not to use my iPhone if I'm not on wifi. For this reason, I am on the highest rate plan, just because 250MB is not enough for me.
The beauty about the iPad data plans is that you are not locked into one long term. You can start out on the higher plan and if you go a couple months without going over, you can switch to the lower one. You can start on the lower one, and if you find that you are using more than you expected, you can switch the the higher plan. If you find you're not using the cellular data at all, you can even cancel the data plan. I would suggest start with the higher plan and see where your usage falls for the first few months. If you are consistently under the 250MB limit, then switch to the lower plan and save $10/month.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.