Condition being equal, and cabin size not being the priority, the older (straight back) 182's are faster, lighter, and better performers. Particularly with a STOL kit, they are amazing. The old 182A I used to fly cargo in, was my all time favourite. The standard carburetted O-470 is not an engine which is working hard to develop it's power, and will run for a long time if cared for. It will run Mogas happily in a 182, which may become more attractive in a world where 100LL is becoming less available.
The 182 RG is a delightful plane, and a deceivingly good hauler, though does not adapt well to short rough runways. Expensive to maintain, particularly if terminating action was not done for the landing gear AD.
Depending upon the model of 172 the engine could be simple too, or not so much. The "H" engine version (172N, I think) did have some engine challenges, which might be best avoided. The Continental IO-360 equipped version flies very nicely with all that power, particularly on floats, but the engine must have 100LL, and works harder. Certain maintenance costs associated with that engine are higer than the O-470 equivilent, though that's not a reason to dislike the plane, just be sure you want that engine, if you're buying it, 'cause it might cost a bit more to operate. There's nothing wrong with the older Continental O-300 powered version of the 172, they're just a little older, and require a little extra care.
My personal opinion is that the 172 RG is not worth the extra cost to purchase, maintain or insure. You'd be better spending that extra money on a straight 182 instead, and using the maintenance and insurance savings to cover the extra fuel cost. And you can really haul a load if you have to for time to time.
If you slow the 182 way back in cruise, they're nearly the same fuel burn as a big engined 172 at high power. Back when I was even less rich than I am now, I used to have free use of the 182 RG, but I obviously bought my own fuel. I used to fly around on the lowest cruise settings to keep the cost down on long trips, and was very happily surprized.