I prefer 3Gb versions cause they are cheaper and have smaller ROI time. The hashrate difference is about 5-10% on Equihash, and I'm not sure mining Ethereum on Nvidia is good idea at all.
Now 3 gigabyte cards are not enough for mining some coins, for example, such as Green. With three gigabytes, you just do not start mining. In a year or a year and a half, six gigabytes will be few.