Speaking as, like, you know... not the publisher, I think T20 is much closer to CT than is GURPS Traveller. It's closer to CT than is TNE. The peripheral systems (starship construction & combat, trade, star system generation... that kind of stuff,) are closer to CT than any other edition of Traveller, including MT. Character creation is substantially different, but it works in the tradition of Classic Traveller much more than GURPS does. It feels Travelleresque - more so than GT.
Combat, frankly, works better in T20 than in either CT or GT.
However, the GT books are worth owning. The Far Trader commerce system is way bloated for my tastes, unless I'm running a purely mercantile campaign - and even then they're incredibly difficult to use without spreadsheeting everything. Much the same applies to First In's star system generator, the most detailed (and realistic, to my eye,) I've seen in an RPG. But man, it's a lot of trouble to use, too.
The GURPS Traveller book has a lot of setting in it, though - and the T20 book has almost no setting. If you're running in the OTU (Oficial Traveller Universe,) you could hardly do better as an introductory product than the main GT book. The line as a whole is excellent, but of varying utility if you're not usi9ng GURPS. First In and Starports are especially useful for any system, Star Mercs less so.
My personal rule of thumb is "if it says 'Traveller' on it somewhere, buy it."
