WEG Star Wars here, I looked at the D20 version from WoTC, and found it wanting...a lot. (and I do mean a LOT.)
For my analysis, I'd start with the advantage of a game system simpler enough that you can teach a NON-Gamer with ZERO RPG experience the WEG system's D-6 pools up to the point of actually building a workable character on the fly in under an hour. WOTC's D20 system is a bitch to teach by comparison, and applied to Star Wars, you're often, imho, relying on familiarity with D&D(20) systems.
Second is: it's more flexible than D20-the rules are simpler and provide enough room and cover to run or play or build almost ANY science-fiction without having to go to GURPS level of detail. We actually slipped Kzinti from Known Space into a Star Wars session, the build rules for designing new races were simple enough it didn't take all night, and the modification rules to things scaling from personal weapons to Space Stations had enough wiggle room to deal with Kzin gravity-polarizers and relative damages caused by exploiting RL Physics.
Third: the core book is thinner, which means even if it were printed today, it would be significantly cheaper than ANY D20 product with as much functionality at the gaming table.