I would go with Rock Tomb. Here are my reasons:
1.Rock slide flinches the opponent only if it hits first. (Source) With aggrons base speed,I doubt that he will be able to flinch any Pokemon.
2.Rollout fixes you into using it for five turns.It could be useful if you EV train it in defense so that it can take a few hits until it finishes using Rollout.However,if your opponent uses a special attack then you are doomed as Aggron has terrible sp def.
3.Rock tomb will always reduce you opponent Pokemon's speed by one stage.(Source)This could be useful because since Aggron has horrible speed,it may be able to lower the opponent's speed down to Aggron's level.And even though it has less power than Rock Slide,it has more accuracy.If Rock slide misses in the most crucial of times,you'll be in trouble
Hope I helped!