Both cards (3870 and 8800GT) have merits.
The 3870 uses less power, has a decnet cooler, and has HDMI output which includes 5.1 sound. It is also cheaper and should have a better short-term supply chain.
The 8800GT is noticeably faster for games. It's main weakness is the poor cooler, but it is single slot so will appeal to more people.
The HD3870 beats the 8800GT in almost every area bar the most important -performance.
For this reason alone the GT will be more popular. I like the featureset of the 3870, but the reason most people buy high end graphics cards is to play games. The 8800GT simply plays games better.