That's a decision that you need to make, dude. It really is mission-critical, so have a think.
Okay... I've not used a 2010 Mini myself, but the consensus appears to be that it will play Blu-ray quality video provided that you strip any HD audio down to the core stream. Let's move forward as though that were the accepted truth, for now.
The Plex interface is great. No two ways about it. It's a fork of XBMC, which is a long-established and very stable, attractive media front-end. That said, you'll have to access Plex through OS X to reach this front-end; coming from a dedicated media streamer background, I find this inelegant. I'm accustomed to switching on a device that boots immediately to my media library. You may not consider this an issue, however.
The video quality that the 2009 Mini produced, in my opinon, was good but not great. Sharp enough, but the colour reproduction was less than spectacular. Not bad, but I've had much better... perhaps this is why the degraded quality of transcodes is more apparent to me than other folks posting on this topic?
And let's not disregard the matter of cost. The Mini will cost you £650 with a 320GB hard drive. A Popcorn Hour A-200 will cost you about £170, and you can add a 2TB internal drive to it for a further £80... for me, if you don't need another computer and you're only looking for a media solution, it's a no-brainer. The picture quality will be better, the codec support will be wider, and the bitrate ceiling will be higher (110Mbps, with HD audio).
So, I'll ask you again. Does it need to be a computer...?!
EDIT: These are the matters that come to my mind, in this decision. Are there important considerations that I might've missed?