I cut my teeth on d6 Star Wars- before AD&D, Earthdawn, Shadowrun, Changeling and V:TM devoured my high school roleplaying experience... one of my friends collected quite a few of the books, and I ran two campaigns, including one using the Operation: Elrood adventure module, which I still think is one of the best adventure modules ever made for any RPG, and a delight for Star Wars.
That said, I prefer d20. Why?
1) Ease of preparation/balance. One of the great innovations of d20, IMHO, is that it provides a ready system for encounter balance that most systems, especially point-based systems, lack. For a DM like me (no fudges, dice land where they may, NPC's are as smart as the PC's, I'm NOT on their side), this is crucial. d6 could be extremely hard to balance in this respect.
2) Power levels. PC's in the d6 version of the game started out a million miles from the power level of the main characters from the movies, and given the suggested character point awards, had no hope of achieving their level of skill. On top of that, balancing encounters for such a party was a nightmare to begin with, as would be insuring that characters wouldn't be min-maxed in a ridiculous fashion if you decided to start them off with additional XP's. In d20, on the other hand, I can tell everyone to make 8th level Jedi, and commence the buttkicking. Players have characters with abilities that meet or exceed those of their screen counterparts.
3) Jedi. Jedi are powerful, and it is now possible to have PC jedi with skills on par with Luke or Anakin.
4) The combat/skill system MOVES. The first time I DMed d20 Star Wars, the party invaded a palace, ran an elaborate chase scene, battled a legion of soldiers, fought an epic lightsaber duel, saved a queen, and even saw one of their own fall to the dark side. I saw more Star Wars action in that session than in an entire campaign of the old system...
5) The Dark Side- it's much more tempting, yet not unbalanced. There's actually a desire to go over, rather than to just accumulate a few dark side points to get the stat bonuses, which I found ridiculously unbalanced in the old system (our especially lucky Jedi managed to get +4d to all his force skills through the Dark Side, and kept from going over- and watching him roll 14d on his lightsaber checks, when his control and sense skills were only 1d each, was quite painful...)
There are a few deficiencies- I still don't like the starship/vehicle combat systems, perhaps even less than in the d6 system, and the sourcebooks are much more condensed (though not focused on the novels, which is nice for a guy who doesn't care much for them...)