Don't fall for the 2GB piece of crap of a 6670. Not only is it massively slower in the memory department (Its also clocked far lower, we're talking like 1.3 GHZ to 4.8...), the 6670 will run out of horsepower LONG LONG LONG before you even get close to 1GB, let alone two.
A 6670 is rather disappointing for what I would say is a rather nice gaming build for you. The 6670 isn't the best for gaming. What is your budget?
As JJ1217 has stated, DDR3 memory is slower than DDR5 memory. As for the larger amount of memory (1v2GB), a higher amount of memory is usually better for screens with larger resolutions as those require a greater amount of video memory. Think multi-monitor setups.
I do agree with JJ1217 that the HD6670 may not exactly do your rig justice but, if budget is a constraint, I can understand.
As to answer your question, the 1GB DDR5 would outperform the 2GB DDR3 one and that would be the one to go for.
P.S. Your monitor's resolution would also help others out as well in helping you.
I think I'll go with the GDDR5 unit, it's only 30 bucks more or about. This will be my first time using Radeon video cards, all my others have been Nvidia, my current desktop has a GTX 430 OC and I have used the 9600, 9800, 8400gs, 8800, even a little 2600 years back in my very first build. I like XFX because of the lifetime warranty. Would there be a better card in your point of views in or around the range of 130 max? But thanks for all the reply's thus far.