Firstly Aggron has more resistances, this is huge both with revenge killing and the ability to switch it in. Aggron has less weaknesses to exploit basically.
As M BLADE said, the extra resistances mean absolutely nothing. His special defense is still so abysmal that even resisted special hits do a whole bunch of damage. Aggron gains resists to Bug, Dark, Dragon, Ghost, Ice, and Psychic resistance, along with neutrality to Grass and 2x hit by Water over Rhyperior. Bug is used only on Scyther who runs Brick Break anyways. Dark is carried on Drapion, (EQ) Spiritomb, (WoW or hp fighting in some cases) Absol, (SuperPower) and Houndoom. (Hi, I attack specially) Dragon is only used with EQ. Ghost is always Special and usually carried with Fighting for perfect coverage. Ice is Special. You're not switching in on Jynx. Psychic is potentially useful for things like Uxie and Mesprit (but they have U-Turn if needed to Donphan), but Alakazam tears through you. Rhyperior resists 0x Electric and Fire over Aggron. The Electric immunity actually helps and the Fire resist lets you come in on Arcanine like 75% of the time. (no WoW or HP Grass)
As far as weaknesses go, Aggron is 4x weak to Ground and Fighting along with a 2x weak to Water. It's also for all intents and purposes weak to Special Fire and Grass attacks. Solid Rock is such a large and underestimated fact that you cannot even put Aggron's resistances on par with Rhyperior's defenses. Nope. Rhyperior is 3x weak to Grass and Water, along with a 1.5x weak to Ground, Ice, and Fighting.
Aggron's resistances mean nothing and Rhyperior's actually matter.
Rhyperior is pathetically slow it handicaps itself (not being able to break 180 speed is ummm) and its defensive checks like Milotic/Leafeon often don't even need to invest a single point in speed to beat/check it if its non Rock Polish (RP still isnt getting past Milotic and the gang). Aggron outspeeds ALL of its defensive checks even without a RP set. That is huge when checking it. Its the difference between able to drive it away and recover and getting 2hko'd because you're slower. That and Head Smash basically eliminates all would be checks for it, again like Milotic.
On offense, I don't mind you using CB Aggron. I just sac something and weaken it a little and then kill it with any Special move I have on anybody. Balance nowadays it seems has one of either Intimidate Top or Donphan to come in and scratch you up or spin away entry hazards. Stall also carries one of those two. SubRise Aggron is 10x easier to switch into then CB Aggron so it really makes no difference if its SubRise.
Lastly who cares if CB Rhyperior does ~70% to Milotic, when it can just Recover/Rest on the inevitable switch out? If you keep your Rhyperior in to try and outstall Recovers or to get a crit, then thats just idiotic. Wee nice way to lose your Rhyperior for nothing and a valuable T-Wave/Electric immunity and try to rely on hax for wins. Im not even gonna say why that is bad lol.
I care a lot. That is easily the coolest thing about Rhyperior. It's not relying on a crit to beat it and you're going to outdamage the recover stall. No, the coolest thing about it is that MILOTIC IS DOWN. Sharpedo, Houndoom, Moltres, Kabutops, Feraligatr, Arcanine, Blaziken, and RPAggron, (yes Aggron) are now able to sweep unhindered along with other NUs that I failed to mention. Me and some people are #stark talked about this a while ago. There is but one perfect Milotic lure in UU, and that is Rhyperior. Once it's stuck at 30% (or less if it tries to recover stall) you're free to tear through teams with their glue effectively unhinged.
(Part of the reason I believe Milotic and Rhyperior to be suspect/broken: Rhyperior's only hard counter is Milotic. Milotic walls all that crap.)