If these are the 06 P bikes you're talking about, the frame geometry is almost exactly the same (the rear triangle formed (I don't exactly know how to say this in biker terms) on the P.2 is smaller than the one on the Cr-Mo). The bikes will feel the same geometry-wise, but the Cr-Mo is heavier. Being steel, it would stand to reason the Cr-Mo is stronger. Pretty much the components on both bikes are the same, but the fork on the aluminum is longer (110 mm versus 100 mm) and the crankset on the aluminum is slightly better. The Cr-Mo has Avid BB5 mechanical disk brakes, while the aluminum has Hayes Sole Hydraulic disk. If your shop can get in some other bike, such as the ones XSL_WiLL mentioned, without you having to pay for it first, you should probably try them out first, because I would have to agree that the new P. frames are a little different. The definitely don't ride quite like the old ones, which totally kick ass!
Of course, XSL_WiLL is going to disagree with me, and I will definitely consent to the fact that there are MUCH better frames out there, but the O5 P. Frames are really good rides! My friend has one, and it is SUCH a good ride.
Anyway, more about your question, if you're on a budget, go with the P.2 cr-mo