SquawkIdent,
The short answer is that the fuel consumption will be similar as the both have (pretty much) the same engine* (160HP Lycoming O-320).
I'd say the 172 is a better shortfield machine.
Load carrying will depend on the particular variant, but I'd guess it won't be vastly different between them. You really need to check the W & B schedule for each aircraft to be certain.
I only flew the PA28-180, which did have a better load carrying ability than the 160Hp 172s, but it wasn't by that much.
If the club only has one 172, then what's the availability like? You may find that its not easy to get when you want it.
Personally I don't think that there's a huge ammount to choose between them. Depending on the club currentcy rules why not get checked out on both?
Brooklands
*A generalisation I know, as some older 172s have the six cylinder 145 Hp Continental O-300, others have the 150Hp Lycoming O-320, and the newer ones have either the 160Hp Lycoming IO-360 (172R) or the 180Hp Lycoming IO-360 (172S)