I just bought my current T this past summer. I had the same dilemma about swapping to either 16" or 17" wheels at first. After some search, it was decided to just stick to 15" as prices are cheaper whether you get them new or used. I think if you have the money, the larger wheels do look a lot better, but the penalty is on the extra weight of them (not including the weight of the larger tires already). I ended up buying an used set of Centerline 15x10 locally from craigslist, and will be using 33" tires as soon as I can get it lifted.

By the way, if you are looking at new tires, I found a site that has some good prices. Please know that I've not use them, so I have not personal experience with them.

http://www.nebraskatire.com/hot_deals/light_truck/lt_truck_hot_deals.htm