I think that comes down to personal choice and how you decide to integrate it with the rest of your home entertainment kit. Personally, I like all my media to be in one place, and have my various devices fetch it from there - so the ATV2 is best for me. (it even bothers me that my cable recordings are 'in' a different box!). Other people don't like having their PC (be it 'normal' or HTPC or media server) turned on all the time, so they liked being able to synch various bits of their library to their ATV1. I was never interested in the ATV1, but I ordered the ATV2 in the first week they were available. I'm not saying the ATV1 people were wrong - just different
It's worth pointing out the little curiosities with the ATV2 as well - like if you rent a movie from iTunes on your ATV2 (connection speed dependent) you can watch it immediately, without having to download the entire film first - BUT - you can only watch it on your ATV2. If you rent a movie on your iPad/iPhone/iPod Touch - you have to wait for the whole film to download, but you can watch it on your device OR your TV via ATV2 and AirPlay. It's not a complaint, as I can kind of understand the logic behind it.. It's just curious for a 'system' which is so well integrated.
Although they are named the same; they're both very different things IMO.