Hi Old Dude,
AGM is a (better) class of VRLA battery, VRLA meaning Value Regulated Lead Acid. In AGM design, the "acid" is more of a paste and held in a Glass fibre mat between the plates. The VRLA and AGM monikers don't tell you the full story, its real trick is that it is a recombinant battery, meaning that the gas (H and O) is recombined within the cell to reform water. Hence, no water lost. No other cell type does this as well as AGMs. All "wet" cells, including the Calcium type will ventboth on cahrge and discharge. In an open engine bay with mass air flow that's ok. In a sealed boot? Not for me. Mazda's engineers decided that even with an AGM battery, that only vents when there is trouble and at additional cost, to put vent tube to the outside as a standard precaution. They did so much good work on the design of the car I'm not going to second guess them.
The vent tube is a safety feature and it only used when the battery is mistreated by the charge circuit (over voltage failure) or a near short circuit. In normal use circumstances the hydrogen and oxygen is just turn back into water in the paste. The valve is strictly a safety feature in case the rate of hydrogen evolution becomes dangerously high.
Over the years I have read most of the battery discussions on this and the other MX5 forums. There are many "views" on what type of replacement battery will "do".
In boils down to money and nature of the ownership of the car.
If you have a "keeper", to die with, garage king like me (he's male) you'll buy another AGM just has Mazda intended for all of the features. Its a no brainer.
If you are just keeping the car "for a while" you'll wound the vehicle for the next owner by fitting a non-recombinant Battery based on price and the fact that you wont have the car long enough to get the value back if you buy the genuine Panasonic.
My ten year original Panasonic is reading 95% charge state capable because I manage it as a part of the servicing duties of owning such a keeper car. It was only 80% odd when I got him four years ago. I expect it to be still good to go at 15 years of age.
More info on a good way to manage your battery life is at
http://mx5cartalk.com/forum/viewtopic.php?f=55&t=33738old Steve