Considering the monthly charges you will be paying for the next two years, an extra $100 should be a no brainer. Sounds like it is definitely worth it.
To OP: I would say so. You can turn around and (hopefully) sell your 3GS next year for some good cash, right in time for the next iphone I hate how they have us (consumers) by the freakin' balls! Screw you, apple!