Finally! A comparison!! Let's kick it off!!!
(btw I'm going to do a mix of competitive use and ingame, basically as many aspects as possible so yeah)
Well, lets look at stats first.
(I can't seem to get the screen shot of teh stats, so I'll just type what I was going to say and put the screen shots in later)
As we can see, Agrron has an impressive 530. Nothing to be ashamed off, of course.
The highest stat it holds is it's very good 180 Defense stat, which allows it to take some beatings in times of need. Also, a quite impressive 150 attack also follows up on it. So it can easily deal out some beatings too. The stat spread screams Physical tank and slaps you in the face with it. if you can make it last, then the opponents won't last. It's also possible to give out a good defensive EV spread, for support moves such as Stealth Rock and Roar . Dragon tail. (I'll go into moves and move sets in further detail later on.)
However, while all the physical side is good, teh special is not. Even weaker neutral moves or high powered resistnt moves are going to do heavy hurting on the special front. And while in-game it could be possible to just about get away with a mixed set, competitively there is no way. The speed isn't flattering for Aggron either, which can only be patched with a scarf and then is still going to be out-ran quite a lot.
Donphan's BST is a bit lower than Aggrons, sitting at 500 exactly. Although this is still not exactly a low amount, it does seem to come out losing. Also like Aggron, it's highest Stas are attack and defence. While it's Hp is sitting higher at 90, it's attack and defnece are noticbly lower to Aggrons (120 for both). It can still take a beating at least physically, and give one out too. But despite this, it's lower attack and defence and still being slower than Erick Pickles in a fun run means it has pretty much lost against aggron. For stats at least..
Aggrons: Steel / Rock
Now, steel rock has 9 resistances and 1 immunity. This makes it a very good defensive typing, in theory at least. it does only have 3 weaknesses. But however, 2 of them are quite bad. 4x weakness to fight and 4x to ground. Both of which are very common attacking types. This means really you have to scout your opponents team quite a lot for powerful moves that can easily take Aggron out. That might be difficult in situations. But if you do, it can be very useful for walling out Opponents. Not to mention a rare resist to Dragons, which Donphan can't boast.
Offensive wise, all is not well. Steel typing as very little use with just 2 types being weak to it. Rock is a bit better, being able to eliminate Bug and Fire aswell. These are just about enough to cover a decent few types for teams. But other than that, little use.
Pure Ground type is decently uncommon. The only other one to sport it in OU is Dugtrio. Now, Ground has just 2 resistances and one immunity. However, electric is a common offensive type, and often in OU, is the main reason to use a ground type at all. Donphan is also the bane of Tyranitar missing out on Ice beam, being able to wall it's stone Edge and killing it off with EQ.
Ground is a very offensive type. STAB EQ is very powerful coming off 120 attack. Mix that with Stome Edge for perfect coverage and away you go! Steel, Rock, Fire and electric, all decently common types, are weak to the powerful EQ. This elimantes a lot of threats going around.
Sets and Moves.
Aggron dos have quite a good move pool. In-game, it get's good STAB via level up in the form of Iron Head. Superpower, Stealth Rock and head Smash are very good moves to get via breeding so that may be a good choice. Stone Edge is a powerless STAB for TMs, and coverage isn't rare. However, almost all the relevant coverage and moves are special or force Agg to have 4 move slot syndrome. So either you can't get the moves you want or can't find any good moves for your set. EQ is good, but Donphans uses it better thanks to STAB and typing. The elemental punches are possible, but don't give it a lot unless you are targeting something specifically. A popular set is Subpunch, but that can leave it open with many holes.
Donphan also supports a quite wide range of moves. However, not only does it sport up SR, but rapid spin as well. This makes it better at utilizing the defense is does have. It also thanks too Gen 5, learns seed bomb. This makes it trickier for Water types to come in and ruin Don's fun. As mentioned earlier, a powerful EdgeQuake combo is very effective for taking out threats on the switch or for dealing heave damage to walls. Not only that, but it manages to learn Ice shard. That I a good priority move, which also allows it to take out Grass and Dragon types quickly or fur just generally taking out weakened Pokemon. Donphans got better Sets :P
I want to add more but i'm close to the limit, so I'll add in sets via comment. enjoy!