That's a REALLY tough decision. I think overall, Crobat is more fun and easier to set up (one energy for the WIN), but for what he has with speed, he lacks in real attack power. poisoning for 4 is AMAZING to me, but its so easy to retreat, level up, or evolve out of the poison. they take a guaranteed 40, but they can maneuver around it. But paired with things like the GE Weezing, miasma valley, and the MT Toxicroak, you can make a hilariously annoying poison deck. If you go this route, tech in some SP stuff and use Crobat Gs for extra damage, and maybe even Honchcrow G to get out your stadiums and whatnot early game. Even try a tech of the UL Jirachi, for late game punishment- they've been retreating their poisoned stuff to the bench to save them? De-evolve them for some quick and easy KOs. This is definitely a fun deck, IMO, but its very easy for some of the really popular net decks to get around.
Steelix, on the other hand, is the complete opposite. He's a big stage 1 who is all about tanking. Expert belt and special metals are your friend here. It has a good resistance of psychic, 140 HP, and some alright attacks. Best of all, he laughs right in Crobat's face- not poisoning this guy, that's for sure. You probably want to run this deck with heavy search and draw cards, and make very good use of felicity's and engineer's. Toss your energies into the discard pile, and pull them back with his first attack. This can be helpful in getting back special metals late game, when previous Steelix's have been knocked out. With special metals and DCE's, you can get him powered up reasonably fast, but if you build this, expect to be focusing on heavy set-up for the first 2-4 turns of your game.
If this is for the long run, you may want to lean more towards Steelix as well, because of some of the things coming out in the next set. Leafeon is going to be a BIG deck, I'm thinking. Leafeon with the UL roserade. They attach their energies to give you 2 or more special conditions, and then Leafeon comes out wailing for 50 for each of those conditions. Steelix completely laughs at this idea, because he cant be hit by special conditions at all!
Well. Didn't mean to give a whole review or anything, but those are my thoughts on those cards and their decks. It's ultimately up to you, what your meta looks like, how competitive you are, and all that good stuff.
Think on it a bit, and LMK if either of these still interest you.