Rhydon: Which one to power up? Suboptimal movesets on both

Unfortunately, I'm going to have to say neither... You pretty much want either Mud Slap + Earthquake or Mud Slap + Stone Edge :/ They are the best moveset for both attacking and defending.

Difficult to say.

I'd power up the first, despite having worse IV because I prefer Mud slap as quick move rather than Rock smash.
Also it's already level 25 so you'll need less candies.

Another option is waiting for a new Rhydon

IMO, Rhydon does not make a good defender no matter what moveset it has. Weakness to water really waters it down.

Rhydons for defending are totally useless. It is useful attacking with rock smash. It is not good at defending because of two "times 1.5625" damages from water and grass, Megahorn is not good without STAB.

Like the fellows above me just said, Rhydon cannot be a defender because Vaporeon tear him apart like a pice of puddim due o this double weakness and everyone have a Vap army these days.

BUT, rock smash Rhydon is the best counter to Snorlax, since it resist most of his attacks, have great bulk and do super effective damage on the fast attack, so i would power him up to use against the big fat cat-like leviathan.

In my opinion Rhydon in def could be usefull if you go with your friends to make high lv gyms ( in this metà is not an efficient way to play, but some like to do that) because he can force a swap and also force to use a water/gras mon that could be useless for every other fight.
If you want a mon to place in low lv gyms ( lower than 7 ) it's quite useless.

Dont power.
Defense on rhydon is useless. Only high cp is good. But 2 double weekness to water and grass makes to easy to kill him.
Use it for attack! But wait for mud stone edge or mud earthquake.
I have one of each this A rated movesets over 3k cp and they are brutal. Even against dragonite stone edge is crazy. And he is also a snorlax killer

Megahorn is not Rhydon's best charge move, but it is still a good move. Mud Slap is Rhydon's better quick move, and is probably more important than whichever charge move he has.

A 2300cp Rhydon with Mud Slap+Megahorn is still a decent defender if you just need more high CP monsters to put in gyms. But wait till you are actually putting it in a gym and power up specifically to place higher than other defenders.

On the final point, Rhyhorns (like Growlithes) are fairly plentiful in desert biomes, and nest changes will not affect that. Before the CP changes prompted me to actually evolve and power up Rhydons, I had several hundred Rhyhorn candies.