Bush just isn't a very good running back. He's good at everything else (returns, pass catching). He's just not very good at running out of the backfield.
What did he average tonight on the ground? About 2.7? That's not far behind his career average.
He reminds me of Vick a little bit. The league's best athlete, but a guy that's almost out of position because he can't do the most basic things required out of his position (with Vick it was through the air, Bush it is on the ground).
I'd love to have him on my team. He'd be fun to put in the slot, put on returns, and hand the ball to 3-5 times on some gadget plays. But he's not a starting running back in the NFL. He's one of the best weapons, no doubt; he's just not a starting running back.
As for the Felix vs. Reggie debate, Felix is the better runner out of the backfield, but Bush is the better player at this stage with his versatility. We haven't seen Garrett creatively use Felix in the passing game, so assuming he could do what Bush does is purely hypothetical.