In my case AGM's seem to have done well. When I say in my case, I am talking about going on 7 years with 2 Optima AGM's. I just pulled them and had them checked at Interstate on one of those high dollar testers that Maine Sail recommends. In fact they showed quite a bit above there "rated" capacity. Unfortunately, I don't have a baseline to compare them to since I didn't have them checked when I first bought them.
Since the boat sits on a trailer most of the time and they have been discharged rarely, usually never below 11.0 - 11.5 volts (?) They have also been connected to a fan running 24 hours/365 days a year for the past 5 (?) years. They have been connected to a 20w solar panel with a Morningstar Duo controller (4 stages).
They have never been "equalized" and only occasionally I throw them on a pulse charger over night. I'm not saying that I recommend the way they have been maintained, only listing the facts.
I realize if you are a cruiser and actually get to use your boat regularly and your batteries go through the normal discharge cycles, your mileage will vary.
Although they have held up well, I got them at a ridiculous price from a friend years ago. I would find it hard to pay what they go for now. They are nice because the batteries are hard to get to if I had wet cells it would be a pain.
They will now probably go belly up.