Another vote for the OTU, or a version of it at least. Our campaign is very much a Spinward Marches one, using the MgT Spinward Marches as the base. Inevitably, and I am sure deliberately, there is so much possibility for different interpretations that, the OTU ends up getting becoming OTU mod 2, or YTU! After all, my feudal technocracy probably doesn't look quite like yours.
In terms of rules, we pretty much use the MgT baseline for careers (sometimes using some of the careers books as well), skills, costs, equipment etc. Interstellar travel is by jump drive as CRB, ships are designed using CRB and HG, use the psionic rules as RAW (but harder to qualify for psionic skills, only 1 in 36 people have any capacity after testing). We have modified the combat rules slightly (see some previous posts, a while ago!), and use a campaign specific list of weapons instead of the mess in the CSC. Aliens as written, personally I have no problem with the standard types, Vargr, Aslan etc.
However, all said done, while we have tinkered with various rules, and fleshed out various planets etc, have basically run a MgT Spinward Marches campaign, and still greatly enjoying it
Egil