HD 5670 GDDR3 is a bit slower than the GDDR5 version. It's got faster bandwidth compared to GDDR3.
So technically, if you're going for one, it's best you get the GDDR5 version. Some of the GDDR3 products may be advertised with 2GB of VRAM, but that doesn't mean it's better. Higher VRAM is only necessary for higher resolutions.