The only downside that I could see with an AppleTV over a Roku was that Roku offers at least one model with 1080p... I believe AppleTV is just 720p.
If that matters to you anyway...
Dave
I think the 720 vs 1080 decision should be driven by basically two things.
1) how big of a screen will you be playing this on? of the screen is 40: or less then 720 could be on the table. At 42" I would start to worry a bit and over 42" I wouldtry to stick with 1080p. Just one man's opinion YMMV.
and
2) What are your other sources? If you have a large screen and a bluray player that you us regularly, the lower resolution of a 720p stream will become quickly noticeable. And while 720p in and of itself is not really that bad, if your frame of reference is the high res output of a bluray watching a 720p stream could possibly become irritating.
Of course one other thing to be concerned with is whether or not your internet connection has the bandwidth to cleanly stream at 1080p. Because no matter how good the picture is, if the stream is constantly pausing because the buffer emptied out, you will tire of it in a hurry.