Have mini, ATV2 and Roku...experiences
Having all of the mentioned hardware, here is my experience
mini (2011) has 8GB RAM and when I am watch either ATV2 (hacked) or Roku, nothing else is running (not even screen saver). FiOS router (strictly on N network, isolated channel but likely the bottleneck in things)...
try to run any high def file (anything blueray ripped) and it usually studders on both Roku (I have two) and ATV2 (I have two)
standard def files run fine, can run and I can stream to two TV's without issue
MKV and mp4 files are hit and miss on Roku (even when connected via USB using USB Channel)
using mini direct to the TV is best (but you only can watch on that TV), I have three TV's
Plex clients are not consistant. Can watch on the mini but not watch on the Roku (same file via USB) and streaming buffers alot
Rippey