I an ATV1 and ATV2, in fact I just picked up a 160GB ATV1 for $120. There are only a few basic differences between them, Netflix, the sports (MLB, NBA) and iCloud music/Photo Stream. Once you jailbreak the ATV and throw XBMC that differences diminish a lot.
Since it has onboard storage that syncs to iTunes the lack of PhotoStream and iCloud music is not a big deal. I have an MLB add on in XBMC so that covers my only sports need and again the onboard storage is great.
The ATV2 will play slighly higher frame rates at 720P than the ATV1 but for my needs it is not a big deal. And if you really wanted you could throw a Crystal card in there for under $20 and have 1080P output. Since most of my current media is from DVD rips, it does not matter to me. It really comes down to what type of media you are playing, if it is iTunes purchased I am betting the quality is the same, if it is DVD rips, there is really no difference, if it is BR rips then the ATV2 will have slightly better video quality.
IMHO to sell an brand new ATV1 to buy an ATV2 does not really make sense, especially if your plan is to jailbreak and add external storage for media.
If you do want to sell it, please ping me, I would love to buy another brand new one.