Well first and foremost, Gillies is an exponentially better skater than Lucic. Like, Gillies has Niedermayer/Gaborik speed, whereas Lucic's NHL future hinges on improving his mobility to "greater than that of a pregnant yak".
As of right now, I'd give Lucic the edge as far as toughness/strength goes, but Gillies is bigger and once he fills out, that combined with his skating ability means he'll have a much bigger impact at the NHL level. He definitely has the mindset to be an absolute wrecking ball on the forecheck.
Neither have a lot in the way of hands/hockey sense. I like Gillies much more as a prospect, he's basically a guaranteed NHLer in my eyes.