I've used both. XBMC begat Plex so they're quite similar and it really comes down to personal taste as well as what devices you use and own.
Plex has a couple of nice features such as 'on deck' which does a nice job of highlighting recently added media. The Plex media server is also great because it organises and downloads metadata about your media automatically and it makes organising media easy and painless. This media server also makes it very easy to run Plex and access your media on a range of devices from the iPad to a laptop to a smart TV. If you use multiple devices in the one home, Plex is worth taking a look at. Some users reckon Plex doesn't run great on a Windows machine as it can prove unreliable. This is probably because Plex for Windows is a relatively new offering. However, Plex runs and looks particularly good on any mac machine and I think it was originally designed with OSX in mind. That said, it looks to me like Plex is slowly moving towards a more monetised offering as evidenced by the Samsung TV partnership and the more pricey iOS app. I also have a feeling they're going to release a dedicated Plex box similar to the Apple TV. It's also worth mentioning that the Plex media server app for iOS runs without jailbreaking where as the XBMC app requires the user to jailbreak.
XBMC is way more customisable than Plex and sometimes it can seem a little more complicated, as evidenced by the requirement to jailbreak iOS devices. I think XBMC has more plugins and skins than Plex and the community surround XBMC seems to be much larger and much more willing to implement new features. That said some of these features or plugins require the user to have a basic understanding of how the OS works. This might not be an issue for most people here but it can be for girlfriends, kids etc. There's also a Plex plugin for XBMC which replicates the features of Plex called PleXBMC. XBMC also has a variety of alternates which people can install e.g. XBMC Live, Openelec. XBMC will as far as I know always be free and open-ended. In short, there'd be no Plex without XBMC and if you a like a HTPC you can tweak and personalise XBMC is worth a look.
Currently I use the openelec version of XBMC. Why? Because it boots within ten seconds into XBMC and it makes XBMC run exactly like a device such as a DVD player. This means I never have to mess around with the OS. As seamless as Plex worked with OSX it couldn't quite replicate this experience. The only real feature of Plex I miss is the media server as I find organising my media a pain. My favourite add on is the Sabnzbd suite which includes Couch Potato and Sickbeard. It automates the process of downloading movies and TV shows. However, I think there's a similar version for Plex.
You can install and run either app for free right now and decide which suits you best. Then there's Boxee, Windows Media Centre, Mediaportal...
Further reading here:
http://www.tested.com/news/xbmc-vs-plex-...emacy/390/
(a year out of date though)