Thanks tAG & Julius! I think I'll take my chances with Spring I guess.
Glest: At one point I fiddled with it and thought it was easy, but as you pointed out, it's to Warcraft-ish (dunno, maybe it would be OK for an Ants vs Thermites -pretty cool actually- RTS, but not much outside of that I think).
Warzone: I've just played it, but I loved the way you could zoom, rotate, etc the camera; it added a whole new level of interactivity. I thought it was the safest choice, but that if the animation code is indeed just targeted at tanks, that's a huge turn-off (I mean, that would rule out Fremens, Sardaukars and Bene Gesserits).
Spring: I thought it was pretty cool, but I thought the documentation/tuts where kind of messy. Regarding the semi-legal stuff, I guess you're talking about the assets, aren't you? That wouldn't be a problem since a friend of mine would be modelling the units (I would ocasionally jump in, though my skills with Blender are quite limited

).
I know Dune is an active franchise (God damn that Brian Herbert guy!), but -disclaimer: IANAL- a non-commercial game would fall under the fair-use I think. Still, I know one should try to make its own universe, but Dune is the only book that makes me think that I couldn't come up with anything cooler than that (I mean, common! Every faction is as cool as it gets: Mentats, Bene Gesserits, Sardaukars, Space Guild)