Well that's an interesting question.
I tend to like low wings due to better visibility in the circuit.
The Cherokee 140 is not the greatest climber in the world, but once going it gets along OK. They have short stubby wings which I quite like. The Cherokee 140 standard seats are pretty awful, especially in the back. There's more to think about with a fuel pump and fuel tank selection, but that does make people fuel aware. I don't like the one door access. I think they are easier in a crosswind than the 172.
The 172 is a pretty robust thing, probably a better outback aircraft (with shade under the wing, more ground clearance etc. Lighter controls than the Cherokee. 2 doors for access. Terrible visibility in the circuit. Not much better visibility out of the circuit either, long wings that seem to block the view. They glide pretty well. The older models (like the M) have that last stage of flap which is like a barn door. That gets you down in a hurry, but I think if people were not aware it might catch people out (especially go around). The newer ones don't have that last stage of flap, so they don't catch people out, but they don't want to come down either (unless very slow).
I'm not a huge fan of either, but in saying that they are both OK, just a bit different. The Cherokee 140 is at the bottom end of the Cherokee range, so if I wanted to fly a few people I'd go for the 172 (or use a bigger engine version of the Cherokee, the 235s and 236s have some power!)
Otherwise, I would probably go for the one that had the best equipment, or the one that looked in better condition. (Such as a brand new leather seat airbag equipped IFR 172 over an old Cherokee 140).
If they were both of the same vintage and with similar equipment, if solo I'd probably choose the cheapest!