I like (& own) both. GURPS Starports is one of the few GURPS products I do own and peruse. Its a gearhead's dream for making a highport & downport. I used it two years ago last when making up the Mertactor/ D-268 B-class Imperial port as a jumping off site for my players.
There is a gearheaded portion in MgT's Starports, but I haven't yet used it. It IS well illustrated with common port-vessels, & a few unique to a certain junk-starport as Mssr Middenface has stated! :wink: 8) 8) 8) 8) 8)
But the GURPS book is deep in detail, perhaps more so than MgT's, and hence its favoritism. MgT's is a recent addition, and has to compete in the niche RPG market. Personally, the MgT one is easier to read, but with the GURPS version out, I find myself looking back for corroboration often, although one of the differences is minor--the GTL vs MgT TL baseline of IMperial Starports varies. GTL its TL-10, MgT TL its TL-12.
Final analysis (& MHO), GURPS Starports more for the GM & Gearhead; MgT Starports more player-friendly.
YMMV