I'm taking RG3. I've said on here a while back that i'm a big fan.
Luck played in a pro style offence at Stanford so he'll be better from the get go and will be the number 1 pick IMO. I don't think Griffen would be in that discussion.
RG3 will IMO eventually be the better QB, taking time to develop. He's accurate with the pass (completion % and TD:INT ratio) and is good on the run. The offence he played in at Baylor though limits what reads he makes so that'll have to develop some before he can get his accuracy etc up. I think he can do it though because he is an intelligent guy (accepted to law school IIRC).
If you want him to start straight away it depends whether you can afford to go through some growing pains with RG3 or just want to Luck to come in and be good straight away.