as far a "trying it out on a mac" that's really not appropriate, as they're both completely different animals on iOS.
plex was a fork of XBMC, it's been re-written with most of the same functionality, but most of the code is now different.
The big difference now is that Plex runs using a Client / Server model.
This means that you must be running a server (windows, OS X, or Linux) to use it. There are clients for OS X, android, and iOS, as well as a newly released windows client (also roku, googletv, and some new LG tvs and Blu-ray players)
The server will take any incompatible files and convert them on the fly to something playable on the client device, or stream the file directly if the client can handle it.
because of this, you can watch a raw blu-ray rip using Plex, I'm guessing if you tried to do that with XBMC, it wouldn't be that smooth.
also one perk of this, is i can watch a program on my living room tv using a Mac mini, i can stop in the middle, and pick up in the same place on the AppleTV in the bedroom, stop that, and move anywhere and pick up on either the iphone or ipad (even from outside my home network, but the picture isn't that great due to bandwidth)
for Plex to work, your server must be running, and depending on what you're trying to stream must have a decent processor in it.
for XBMC to work, you only need access to the files, so if you have a NAS, you don't need a computer running.