How much better than the 2010 version?
It would be nice to, say, stream a video via AirPlay (using AirServer) without having to quit Plex first.
I could sell my current Mac Mini and invest a bit extra to buy the new one, do you think the base model (2.5GHz Core i5) would be a decent upgrade?
The other thing to look into is the processes that are running when you experience stuttering. Depending on the process (and/or app), the answer may be more RAM or just not using that app or process when playing back video. However, for HTPC purposes, I tend to think that 4GB of RAM should be plenty.
Hopefully one of these two areas of investigation will yield an answer to your playback problems. If not, try playback without Plex. I've not used Plex much, and hate to point the finger at it; but that would the next thing to check if the first two areas fail to uncover the problem.
Good luck, and let us know the results!