XBMC is still in early alpha phases meaning it's full of bugs as the developers work on it. You probably won't care for it at this point as it's lacking some features of XBMP (network buffering, AV sync, etc). You should stick with XBMP for now, and wait until XBMC becomes more stable or adds a feature you can't live without. I know for many people (myself included) the AC3 passthru is the feature that makes me want to use it only; however, they still need to implement AV sync and network buffering, then I'll be using it as my primary player (I only stream from my comptuer and only movies with AC3 audio, so you see my situation).
To sum it up, if you're happy with XBMP, stick with it for now. Try a CVS build of it (the latest is stable enough for me, no complaints from me). XBMC will one day rid away with XBMP, but it won't be for a while.