AGM (absorbed glass mat) batteries are in short a different technology to your conventional flooded cell.
Rather than having electrolyte just sat inside the battery it's (as the acronym suggests) material soaked with the liquid.
The benefit is that you've more flexible mounting options (some manufacturers say upside down as well, just they don't recommend it if you can help it) as well as not needing to vent the batteries.
Plus they're maintenance free.
I run an XS Power d5100 in my car. It's good for 57ah and it does the job just fine. I wouldn't get worried too much on the reserve capacity of the battery (the ah rating) and would concentrate more on the cranking amps of the battery as that will be a better indication of how well it will start your car (because after that, the alternator will supply most of the power for the car's needs).
To add to that, the battery (surprising to some) was the first mod for my car, and carried out the day the car came home (within hours infact). It's just the piece of mind for me that the battery itself can deliver more than I'd ask of it even though the reserve capacity is less (this isn't an issue as such given the car has a CTek charger hooked to it most of the time)
AGM technology has now come to the point where all those modified big diesel trucks are comfortably started with the entry level 20ah agm batteries from both stinger and XS power.
The only downside is that the cost of these batteries are more expensive.
As for the brands you've mentioned, I've always been a huge fan of Bosch batteries (though I've never owned one as I've in the past had an Optima RedTop and the aforementioned XS Power) but Varta have has some solid reviews especially in high demand situations such as with car audio applications (one of my friend's has sworn by Varta Batteries in his 2kw audio setup).
I know it's a lot and a quick google search will yield more technical info, but just sharing my experiences with different battery technologies.
