I agree with the above that the two aren't mutually exclusive. Some players have an intuitive grasp of rules systems, and some don't remember which dice they need to roll from one game to the next. Their facility with the rules doesn't necessarily reflect on their ability to roleplay.
Personally I prefer games with deep characterisation and plenty of roleplaying opportunities. However I also enjoy learning rules sytems, and as my knowledge of the rules increases I think of ways to improve my character, or make clever uses of my skills and abilities. Am I a roleplayer or a powergamer?
In one campaign, I wrote several pieces of short fiction about my character, based around his background and his reactions to key campaign events. In other games I've had characters that weren't much more rounded than the protagonist of a video game. I've enjoyed them all - after one particularly intense, roleplay-heavy campaign, it was actually a relief to play something lighter afterwards. I only have so much energy to spend on pondering moral dilemmas and negotiating complex relationships. While it can be fun to explore these through roleplaying, it can also be draining - and gaming is meant to be escapism, after all. There's something to be said for just hitting things from time to time.
As a GM, I've had players who could roleplay and powergame, and others who've not been very good at either. As long as they all enjoyed themselves, and brought something positive to the group as a whole, I didn't really think it mattered much. Often the differing approaches complimented each other, and the art of GMing requires that you offer something to everyone throughout the course of a session.
I see all kinds of nonsense at the Character Optimisation boards at WotC - people dipping into classes here and there just to get a specific feat or ability. However I think the class-based d20 system encourages that in some ways, and my inner powergamer can understand temptation. Sure, some people tune their characters to make them as powerful as they can, but others just can't realise their vision of their character without shopping around for the right mix of abilities. I could argue that's roleplaying, not powergaming - having an idea for a character and crafting it from the materials (i.e.rules) available.