20L will give you much more stable parameters and more stocking options as far as plants and fauna goes. honestly lighting and maintenance will be much easier than an edge, but aesthetically, the edge has an... edge.. lol
I have a 12 gallon Edge, and I agree. It's currently my only tank. I love it, but maintenance inside the tank is sort of a PITA. I'm injecting CO2 and dosing ferts and have upgraded my lighting. I'm getting great growth, and I don't mind the trimming and planting, but doing everything through the little hole at the top and using 1 hand only can be challenging at times. You are also quite limited on lighting due to the design.
It looks fantastic though, and I love it. I now want a rimless for high tech and want to move this back to lower tech. I don't want to get rid of the Edge though — I just want a 2nd tank. I'm glad I got the 12 over the 6 due to height. I have far more room to grow plants, and I can have 2x the fish. The footprint is the same.