Completely depends on what you're trying to do. Low requirement plants vs high requirement. Find the plants you like, determine their requirements and go from there.
I believe the 6 gallon comes w/ a 3watt led (it's terrible) vs the 12 gallon's 5watt led (still terrible). You can buy the 5 watt on ebay and swap out the 3watt but there are much better alternatives. (FYI - wpg (watts per gallon) isn't a good way to measure lights, but knowing above might help your research.)
Let me know if you have any questions, I went fairly high tech in mine.
I believe the 6 gallon comes w/ a 3watt led (it's terrible) vs the 12 gallon's 5watt led (still terrible). You can buy the 5 watt on ebay and swap out the 3watt but there are much better alternatives. (FYI - wpg (watts per gallon) isn't a good way to measure lights, but knowing above might help your research.)
Let me know if you have any questions, I went fairly high tech in mine.