With the Evolite, Rhydon's defense can go up to about 558. Rhyperior gets 394 maximum defense. Like you said, Rhyperior's Solid Rock ability allows him to withstand super effective moves better. But it only reduces them by 25% Meaning that 2X super effective moves would do 1.5, and 4X weakness moves would do 3X as much damage. Neither one has very impressive special defense, so a special attack like surf is still going to destroy both of them. For both of them, all of their 2X weaknesses (except ice) are usually going to be physical, so they shouldn't have too much trouble withstanding them. Rhydon's defenses will still filter out more than rhyperior's ability. I'll use two situations below to show this.
Situation 1-Fighting weakness
Hitmonlee (with 372 Attack) hits A Rhydon and a Rhyperior with Close combat (120 base power before STAB)
Against a Rhydon with 558 defense (maxed out with beneficial nature and due to evolite) The attack will do between 176 and 208 damage.
Against a Rhyperior with 394 defense, (maxed out with beneficial nature) The attack will do between 248 and 292 damage. After solid rock is applied, it will be about 186 and 219 damage.
This assumes you invested full EVs in defense. Rhydon's defenses will gain more of a boost when he uses moves like Curse, so he can bulk up much faster than Rhyperior. Rhyperior takes a more damage from this, but he only gets 20 more HP than Rhydon (assuming you invest a full set of EVs in him, meaning you maxed out defense and HP, leaving only a few EVs for actual attacking. most people will not design a Rhydon or Rhyperior like this. )
Situation 2-Water weakness
A Starmie (With 328 Special attack) uses Surf (95 base power before STAB) on Rhydon and Rhyperior.
Against a Rhydon with maxed 311 Special Defense (maxed with beneficial nature and evolite)The attack will do between 439 and 517 damage.
Against a Rhyperior with 229 defense (maxed with beneficial nature), The attack will do between 593 and 698 damage. But, His ability will reduce it to anywhere between 444 and 522.
This set assumes you invest full EVs in Special defense, which would be a waste, since he won't benefit much from it. Either way, neither one has a chance to survive a STAB surf, especially since the opponent's item hasn't been taken into account.
Even with his ability, Rhyperior takes about the same amount of damage as Rhydon, but unlike Rhydon, he still has a slot available for an item. So if you wanted, you could give him something to help him out defensively. Rhydon gets to use another ability, opening him to other things, like Double edge abuse. Personally, I prefer Rhydon thanks to him being a bit Bulkier, meaning he can take a few more hits, and he's able to build up faster with curse. But They'll end up being about the same, mathematically speaking. Rhydon takes hits a bit better, but Rhyperior has a little bit more he can take, not to mention higher attack power.
-I did the math by hand to calculate this stuff, so there's no online calculator I used.