If everything is paid for by your company, and if you don't need BES ($15 won't get you BES anyways) I would get the 6Gb for $30 plan. It is the best value going and you would be hard pressed to go over that amount.
Actually, I'll be leaving Rogers, so not everything will be covered anymore. So I'll be looking into getting a plan to go with my current collection of blackberry devices that I've aquired over a period of time.
Sadly, I don't really know how well the plans work or what not. I might even get dinged since I don't really want voice, just the data plan.