Scrafty can take more hits than Lucario with it's high defence and special defence, but has a really trash speed stat, almost half of Lucario's. Scrafty's attack is pretty good, but Lucario's attack is one level higher in that category, being 110 base and 350 max versus 90 and 306. Unlike Lucario, Scrafty's special attack is very weak, but doesn't naturally learn any special attacks whatsoever, so keep that in mind if you're trying to teach it any special TM's.
Moving on the typing, fairy type isn't in Black 2, that gets rid of one of Scrafty's gaping weaknesses (x4) and leaves him with only two weaknesses (Flying and Fighting) whilst Lucario has three (Fire, Fighting, Ground) but I would still give the type advantage to Lucario because of all the type resistances it has (8, including x4 resistance to bug and rock) not to mention poison, which won't effect him. Scrafty has a mediocre three resistances, with x4 to dark and psychic doing nothing, which would help against the Elite Four's Caitlyn, and both Pokemon would excel against Grimsley, the Dark type Elite four member.
Overall, I would choose lucario because of the superior typing and more balanced stats, and also he has a much cooler design in my opinion :)
I'm sure I missed plenty of points and probably rambled a bit, but this is my first post and I'll try to improve from here. I'll take constructive critism to help me improve on these "vs" posts.