Like you, I was not unhappy with my old BB (a 7290), although I knew when I bought it (switching from a Palm m515 and a separate cell, a Sony E. T610) that I was sacrificing a number of conveniences. When I caught wind of the 8700's features this summer, including the increased memory, faster chip and a number of other gadgety niceties, however, I rushed out and bought it the first day Cingular offered it, and at full price. (It still fulfills my initial two-year contract, but that probably wasn't an issue since I'm happy with Cingular...)
I suspect that when T-Mo offers it in a month or so, you won't find much of a deal on the price for awhile and, if you're flush with Christmas dough
, I'd just buy it from T-Mo so you know you're what you're getting. I consider my 8700 a significant advance in RIM's BB/cell units: the screen is significantly brighter, Web browsing is fast and efficient, and I no longer feel like I'm "missing out." While I don't store the amount of e-mail you do, I can assure you that this unit's memory handles it well and quite quickly (v. my old 7290).