You're both right... I'd like to use something other than Blizz because I feel there are other UF that convey the information better (and include Party Targets). However, I don't feel the difference is that great and am therefore concerned about memory/cpu effects of adding a third-party UF addon. My take from Funky's post is that I shouldn't expect to see a whole lot of bad (or good) from using another UF. In an ideal world I would have hoped that using a UF without portraits for example would free up some cpu cycles but I guess it is truly minimal.
Well non moving portaits are just a texture, moving ones would use resources to display. But nothing that could be called anywhere near significant, unless the addon had a problem.
It terms of UF performance should be exact same whatever you use, since you need to register the same amount of events and run the same functions. It would be possible if the Blizzard UF was coded like shit, but it isn't that bad. I'm also a Bliz UI fan which is why I made a basic addon to just move the blizzard frames, BasicUF on branches. You can find others also that give you a 3d portrait on it and such.