AGM are available in other brands besides Optima, (ie; Interstate, Deka, Delco, Duralast, Diehard, etc..) it's the same old debate in that there are only a few actual battery makers, and most are just rebranded of one of those (ie; Johnson Controls). The Optima circular cell design is somewhat unique and they are perfectly fine lots of my car guys use them and love them, but to your question on sitting for months, AGM will definitely sit longer without a charger/tender hooked up and retain more of it's cranking power than a flooded battery of same rating.
I have the
Deka Intimidator dual terminal 34/78 AGM in mine since Jan 2017, I do keep an AGM tender on it during the winter months when it doesn't get driven that often, but don't bother in the spring summer months. For the price, I've been very happy with it, and I had one instance where my alternator failed and I literally drove the car home about 12 miles, and even with the electric fans, and EFI, fuel pump, etc...it made it home before the battery volt dropped below 11v. It was only about 2 yrs old when that happened, FWIW.
It's not the highest in terms of RC, but there are some others at the higher price points that do.
As per usual if you're limited to a specific GROUP size for installation, that limits some capacity, but if you can fit larger sized battery in your (ie Trunk) then you have decent additional options.
These days, any sub $200 battery that can last 4-5+ yrs is a bonus IMHO, (my 2015 Corvette original ACDelco battery died almost exactly at 5 yrs, note it wasn't AGM and I replaced it with an Deka AGM as well in 2020, so we'll see how long that lasts) but I would simply say if you plan to let a car/battery sit unused for months at a time either take the battery out, use a tender, or at least install a shut off switch so yo have no chance of any parasitic drain on it.