As uzurpator wishes to have his topic free from side discusssion I started now the dedicated topic. (This is a personal view and understanding of them, you can correct me keep in mind, if I did something fundamental wrong in the review it may be the problem of your PR)
First, where do we get the Engine(s)?
Uzurpator Engine - non existent - I call them UzuEngine because I am lazy
TRoS - http://tros.ath.cx/subversion
is a addition on top of OpenGL, in it current state it looks like that you still need to handle quite a bunch of OpenGL stuff yourself. You seem to have all builddepencies (except nvidia cg) in the svn so a compile should be possible without much problems. Some signs of Network support spotted, aswell as MySQL support. Personally I don't know why an engine needs MySQL support. In TROS there is already a gui system. Currently from outside you can't yet see the goal or the design of TRoS when you want to use it.
The license for it isn't yet clear. Consindering the TEmpire agreement that the Game should be GPL the decision useing TRoS engine is astonishing.
Nothing really to review yet, from the said specs, it should be quite good on multiprocessors, this can create subtile problems however. uzurpator promised it shouldn't by design. Currently we don't have much to see.
It will have the necessary base, aka 3D Support, Network, Multithreading, GUI, Terrain rendering
As it's an engine directly for TEmpire it may be better in code size and overhead.
Uzurpator writes it together with the game core, but still seperate so the core can be used together with TRoS. Uzurpator made a deadline: "16 of june 2007" for a "running TE alpha" with thread, modifable terrain, and a some bits of a GUI
The license for it isn't clear either as we don't have seen code. We can only speculate if UzuEngine will do what TEmpire need.
Both Engines looks like useing OpenGL exclusivly. As XeryusTC pointed out atleast TRoS is fixed to it. The OpenGL experience on low end hardware may be crippled on Vista.
Other Engines you may want to look: