Well mine are VIVOSUN brand. If you normally grow in 5 gallon buckets I'd go with their 7 gallon pots, and they're cheaper for a 5 pack for some strange reason. I went with the 5 gallon fabric pots from VIVOSUN because I didn't know they were only three gallon when compared to a 5 gallon bucket. That being said I bought them so I figured I'd try them out.
I normally roll with 4 plants in 5 gallon bucket but I did 6. I suppose the plants will be smaller so I'll be able to fit more. If you're looking at buying VIVOSUN pots then I'd go with 7 gallon or once size larger, if you don't mind wasting soil. I'm not sure what your goals are though. I believe my autos reach their full potential in a 5 gallon bucket and I can only fit four... I have a 1,000 watt HID lighting system.
It's all in what you want I think. I've never used anything less than 5 gallon buckets. If you're trying to maximize the plants true ability then go with bigger pots, if you want to grow more strains and not have plants reach their full potential then go with smaller pots.
You new to growing?