This completely depends on personal viewing needs.
I have both the mini (with xbmc) and apple tv.
Apple tv's biggest limitation is codec support and content organization. The former issue is not a biggie since a lot of content today is H.264 encoded. The latter can be a serious limitation if you want to do something as simple as organizing your personal home videos by date. Apple tv has a very flat organization scheme
Mini (with xbmc) lacks a UI that incorporates a movie rental service (iTunes store, Vudu, etc). Even the netflix plugin for xbmc is so-so. Sure, those folks interested in watching new releases can easily purchase a movie thru iTunes. But that UI is not optimized for TVs. Also, its outside xbmc.
I would prefer not having to use a keyboard and trackpad when watching TV. But thats just me
Overall I prefer the convenience of apple tv. With the new software update allowing smooth streaming of high bit rate H.264 encoded 1080p content, I give a slight edge to apple tv for my personal needs (organized in order of decreasing priority) -
o ability to rent new movie releases
o stream music via iTunes match
o control music via an app with TV turned off
o netflix access
o access to home videos (off camcorder)
o photos
o access to movie library
I also like the fact that any movie purchased from the itunes store now sit in the cloud. What I don't like is that the movies are not blu ray quality and that they are DRM protected. The latter can be an issue if one ever disbands the mac ecosystem and goes with some other vendor
I have come to realize that no solution out there meets all my needs. Have tried out sony's google tv box, WDTV, roku, boxee box, apple tv, mini with xbmc, apple tv with xbmc. All these solutions are flawed. If one device has good codec support, then it will either have a slow UI or lack a good iphone app for remote access (wdtv and boxee). If one has a good ui and codec support (mini with xbmc) then it will lack good remote access.
Sigh!