Depends on the state of the components on the PX. Like Matt, Id go for the PX, for the reasons he mentioned and also because I believe you, coming from a mtb back ground, would maybe be more suited to the larger 26" wheels. Old bikes like the PX appeal to me more than the new super generic 24s you see everywhere these days. The PX will have much more character! Although may be dripping with issues, like, a shit pick up on the hub, sticky magura pistons, tubing on the verge of cracking, narrow tyre-folding rims, begging to be snapped 25.4 BAAAARS AHHHHHHH!
For sure, the 24 will feel more solid, more confidence inspiring with regards to the drive train and brakes but the geometry difference between the two wont be wildly different, after all, 24s were designed around streety 26s! Infact, after Edd Tongue tried 24" wheels on his Pashley, the Ashton Edd tongue frame was born!