As somebody who has seen a lot of both, I'll give the slight edge to Gibbs. Granted, I've also been on the Ulis bandwagon for a while, and I would probably take him over Adams and Purvis, but in todays game, it is hard to over-estimate the value of a guy who shoots 44% from three on seven attempts per 40 minutes. That sort of volume and precision paralyzes the defense, and at Seton Hall, playing in the Big East, Gibbs was the offense. To shoot that well despite being the focal point of the scouting report is fairly remarkable, and there is a strong argument to be made that the Big East - particularly when eliminating Kentucky from the equation - was better than the SEC last year.
Conversely, Ulis was typically option five in an offense that united a pair of ferocious rim-runners with capable perimeter shooters. I suspect that his season percentage (42%?) might not be indicative of his shooting ability at this time. Last season, particularly in conference play, he was very reluctant, and as defenses guarded him differently, his percentages plummeted (36% from the field in conference play, 35% from three).
Floor vision is another thing, and although you would have to give the edge to Ulis, who demonstrated remarkable poise as a freshman, his assist rate was only marginally better than that of Gibbs', who was playing with the handicap of an inefficient Seton Hall offense.
Gibbs has rated poorly defensively, and Ulis, for the most part, has held up well on that end, despite his size. But there were instances where Cal was forced to pull Ulis from the game because of the size he was giving up to an opposing guard, and in being flanked by potentially the best defensive front court in the nation, Gibbs will likely be asked to play to scheme rather than ability.
I imagine both fan bases will choose their guy. Gibbs is a transformative shooter, while Ulis is a bit more well-rounded. Accounting for the context of our roster, I think we're better off with Gibbs.