There are many good AGM batteries out there, if they are treated well. My two favorites are Lifeline and Odyssey. If going AGM on my own boat today, and cost was not an issue, it would likely be Odyssey.
Lifeline's have the huge benefit of being able to be "equalized" or "conditioned" where other AGM's do not. This can be a very important feature.
Deka / East Penn licenses their AGM and GEL technology from Sonnenschein in Germany and builds the batteries here in the US. West Marine and Sam's Club both sell them. Of course at Sam's Club a 6V AGM or group 31 12V are less than half the price of WM.
Many other AGM's are built by companies like Full River in China for big name brands who mark them up and churn them out. If you stick with Deka / East Penn, Lifeline or Odyssey you'll be doing well.
There are drawbacks to AGM however and they ideally need to be installed as a "system" not just as a battery. I see PILES of AGM batteries die well before wet cells and well before they should because people have not taken the time to install them properly or to treat them properly.
Heck just yesterday I quoted a customer on replacements for his four year old Lifeline GPL-4CT 6V's. He has done everything right in terms of alternator, charger etc. etc. EXCEPT he is on a mooring and the batteries rarely if ever get back to 100%. He's killed a $1400.00 bank in four years. I also quoted him on solar panels so this won't happen again.
Today I worked on a boat with a stock 50A alternator, no solar and single stage ferroresonant charger so old it should be a boat anchor. The two 6V batteries are 7 years old, by date code, and just now looking like they perhaps need replacement. He paid about $75.00 each for them. This 7 years despite the owner doing NOTHING to care for them. He never added water and only charged them to full once in the fall and spring.
BTW, AGM, GEL and FLOODED batteries are all LEAD ACID batteries.
Feel free to read this for more on AGM batteries:
AGM Batteries - Making The Choice