Well, obviously FXP is the unofficial version that got released before the official nightlies started to roll out. The cool thing with FXP is that they are collaborating with Sony, and their Free Xperia Project Cyanogenmod builds are the result of that collaboration. They can get drivers, blobs etc. much faster and easier than most other devs out there, so FXP is an awesome project.
Official CM is what you're used to. The nightlies are updated more often than FXP's build, so bugs get fixed much much quicker.
You should probably go with official CM. I can't really see why you should flash an "old" build (FXP)
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