As part of my team I always like to have a Rock Pokémon. Before now I've always gone with Geodude, but I was thinking of switching for Onix for SoulSilver. He seems to have the advantage of speed, however I'm not too sure how the stats match up in comparison to Golem at the same level, and I like that they are dual types (although it can be 4x as devastating when against water/grass etc, obviously).

What Rock/Ground Pokémon do you guys tend to go with, if any?

I'd say evolve an Onix into a Steelix. Metal pokemon are only weak against 3 elements and get half damage of most of the others.
 
edit:

Flip a coin. I prefer Onix over Geodude, but Geodude's evolve forms are sweet.

Onix.

onix kinda sucks, i used one until the elite for... and you'll only get the metal coat pretty late to get a steelix... 
 
so i'd go for geodude

Geoduder has a way better name.

Good points made RE Steelix, but I had no intention of evolving Onix were I to choose him; I'm more of an attacker than a defender and Rock moves are super effective against more types than Steel. 
 
Think I might go for Onix this time though, for a change. I guess it's boring to have the same Pokémon every time.

Onix is badass for the TCG. 

Geodude is a punk-ass bitch, kid. I mean, a floating rock with hands? What the fuck is that?!

" Geodude is a punk-ass bitch, kid. I mean, a floating rock with hands? What the fuck is that?! "

I usually don't use Pokemon with 4x weaknesses unless they are great sweepers (Garchomp, Salamence). I've never used a rock/ground type Pokemon since they usually don't have the Sp. Def. to take a 4x hit which is the route the opponent will take. If it's just for the game, Geodude is a better choice since you're not going to evolve Onix to Steelix even though that's a better Poke. The great thing about Steelix is you'll have access to those rock moves with the advantages of having a steel type.

" I usually don't use Pokemon with 4x weaknesses unless they are great sweepers (Garchomp, Salamence). I've never used a rock/ground type Pokemon since they usually don't have the Sp. Def. to take a 4x hit which is the route the opponent will take. If it's just for the game, Geodude is a better choice since you're not going to evolve Onix to Steelix even though that's a better Poke. The great thing about Steelix is you'll have access to those rock moves with the advantages of having a steel type. "

You will lose the STAB (1.5x) that it would have gotten from being Onix, but the fact that you trade it for better resistance is worth it. Plus, the power that Steelix has kind of negates the STAB to some extent.
 
As for moves that are not of the same Pokes type, a super effective hit will always be better than a STAB for a normal hit (2x vs. 1.5x).

" onix kinda sucks, i used one until the elite for... and you'll only get the metal coat pretty late to get a steelix...  so i'd go for geodude "

You can catch Steelix on Route 47 and Mt. Silver.

I never saw the use in a pokemon with two very common 4x weaknesses to counterbalance a high defence stat. If you have to use either, pick Steelix. Way better typing. Golem is dead the second a water or grass type comes into play.

Geodude has the best name, but Onix all the way.

Dude get an Onix and evolve it into a Steelix.
 
Rock pokemon have to many goddamn weaknesses to be useful!