Sorry for the late reply. I'm using XBMC on a Linux Mint 14 PC with a AMD Radeon 6450 passively cooled graphics card. Performance on my dual-core Pentium D is no problem for media server tasks. 1080p HD videos run smooth. In addition to XBMC I run a ssh server, a low footprint web server (for streaming movies to iPad and smart phones), and use the media PC for backing up my main PC using Luckybackup. I haven't seen nor tried XBMCbuntu but in your case, why don't you try it? It most probably will save you time and hickups when trying to install Linux and XBMC on their own (I had to tweak some graphics related settings to get smooth HD performance).
On a general note about your Ubuntu 14.04 and Xen 4.3 trial: Unless you are a experienced Linux user who can fix problems or find work-arounds, go with regular releases and not with beta versions.
My experience is that installing and tuning Linux can take some time, but once everything is running its performance is at least on par with Windows, but in general Linux is a lot more reliable, particularly over the long run. Actually, the best you can do is choose a long-term-support release, get everything you need up and running, and be happy for the next 4-5 years. You will have a system with no or next to no performance degradation, no viruses or malware (provided you stick to basic security rules and always update the system), and little or no downtime or need to ever touch or fix things. The only regular maintenance task is backup, and there are plenty of free utilities for that.
Whatever you choose, good luck.
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