I like brunettes don't know why...

Regarding Reaction the biggest stumble I've found so far is that there isn't much documentation! The original ClassAct release on Aminet has some stuff, also the NDK3.9 has descriptions on each class and there are some examples on the DevCD. Should be enouph. There's also a PDF tutorial somone did wich seems more oriented towards OS4 but should be usefull.
MUI docs on the contrary seem to be very good.
I've only read part of the intro on the ClassAct release and it seems every class on Reaction/ClassAct is a boopsi object, wich makes it usable from any other Boopsi object. Someone above mentioned that to use Boopsi classes on MUI you have to encapsulate them in a MUI class so maybe the result is the same, does it work the other way around too, i.e. can you call a MUI class method from a standard Boopsi object ?
Pah, I guess the best way is try both. I might even do some benchmark program thing.
Granted I expect each one to have it's advantages/disadvantages depending on each context.
BTW, regarding complex GUIs with Reaction, I've recently read the Boopsi chapter on the RKRMs and it seems to me that a lot of people jumped into MUI/Reaction without knowing Boopsi, I think if you know how interconnection Boopsi classes work you can do very complex GUIS pretty straightforward, maybe it takes a bit more code to set up your connection objects but it didn't seem that much. And it's very flexible.
Anyway, when I get the time I'm gonna try both that's for sure...