well I say 12v instead of 6v because you can eliminate connections. When you connect 6v batts in series to make a set that produces 12V anyway, you can eliminate at least one connection between each set, it is much easier to just have 12v to begin with. Cable cost money and the connections have to be maintained. This is not to say that battery banks consisting of 6v batts do not have a place, they certainly do in larger systems. IMHO one should just cut to the chase and go 12v to start off with.
AGMs have their advantages and disadvantages. It depends on what fits your usage. I use FLA instead of AGM, but with quality AGMs, a charger/converter with an operator interface and temp sensor, you would potentially a really good system.
Gell Cells are OUT, they are not fit for our applications, the players are FLA, AGM, and specific types of lithium batts. Lithium is just too expensive for most people. This leaves FLA and AGM.
AGMs can be deep cycled, deeper and more often than FLA, and they are spill proof. So doing the math AGMs may give you more with less weight, and expense than FLA. You have to do a comparison to get the answere. But this requires some idea of what you want to accomplish in the end.
If you would like to share what you envision that you want your system to do for you, I might be able to guide you a little. I have some links you might want to read that will help you.
