dylan1016 :
Is it opinion based then? theres not really one that stands out from the rest? better features, overclocking, and so on
Of course its based on opinion. In my opinion, price plays just as much of a factor. I'd take a reference 670 over some super-pimped OC version that cost $100 more. In the end, they're both 670s and will perform virtually identically.
The 4gb cards are significantly more expensive and have arguable benefits. They've actually been shown to be a little slower than the 2gb versions in situations where less than 2gb of vram is being used (95% of the time). Games that utilize that extra vram include Skyrim (modded), Crysis 3, and Tomb Raider, so if you're really into one of them than it might be worth it.