The Nike+ is much cheaper, no doubt. I've never had one so can't speak to it, really. I occasionally run with a guy who uses one, and it rarely matches the mileage my Garmin shows when we run together. Not saying which was right or wrong - although I do have my opinions
. (In fairness, it wasn't off that much)
I'd just read up on both and decide if the extra $100 or so is worth the added features for you. For me, it was, and I don't regret it for a second.
Good luck in your purchase, whichever you choose!
-S