Which would you use to claim a gym?

I'd say #2. It's easy to prestige and will sit high.

It depends on the Gym.

If it is a gym that I know flip flops often... I will usually throw in a freshly evolved Eevee that I don't care about.

If it is one that I think might grow I throw in something to dissuade attackers. For me that would generally be one of these three.
1) 21xx Blissey
2) 23xx Snorlax
3) 26xx WG/AT Vaporeon

I'd never throw a Rhydon (or Gyarados) into a newly aquired gym.

depends on the biome, I wreck any and all rhydons in gyms around me with any water or grass type. My 2949 vaporeon with hydro pump rips through them. So for my biome definitely the vaporeon.

Nice rhydon though. My 98% learned rock smash megahorn....

If you are going to build it up right away, then the Rhydon is a bit easier to prestige against. ... Also easier to take down though so if you are not going to take the time to build up several levels, then put in the Vaporeon.

Vape