The DDR3 will be faster in some games, yet Skyrim is more of a CPU intensive and doesn't rely on graphics all too much. Not all games are CPU intensive. There are alot of graphics intensive games out there and more often than not a game is going to rely more on graphics than CPU, so I would go with the GDDR5.
What kind of games are you going to play?
If you're really really tight on moneys, then the DDR3 will suffice. It will not make too much of a difference, but it will be noticeable.