Again, and opinion from me, as there are people on here that know far more about the finer chemistry of batteries, but I did do a fair amount of research before deciding on mine.
I ended up going for a 200ah AGM. It was a Lucas from the local battery shop. Not marine specific, and cost me 200 notes.
So, here's why I decided to go AGM.
AGM is a bit more robust. It has a lower internal resistance, so charges more efficiently. If you are using solar cells, this is obviously helpful as you don't want your precious charge being resisted. They can be used for big power draws, so you could, in needed, use any of your domestics to crank the engine. Finally, they have more "usable" capacity, so pound for pound, you get a bigger battery - i.e. if you needed to (probably not advisable) and AGM will discharge way further than a gel or flooded battery.
Somebody is probably going to come along and argue, as is the nature of a forum, but that's what a couple of weeks of pondering led me to understand.
Wing 25 - Real men have long ones.