First of all, your house battery bank and chassis battery bank are two separate items. You don't have to go all AGM or all flooded. I have a mix of AGM house batteries and flooded chassis batteries in our coach and am a big fan of AGMs. Your inverter-charger will be set for ther house batteries, which is what take the charge. Your chassis batteries aren't handled by the inverter-charger unless you have a Trick-L-Start or BatteryMinder installed. Either way, those small charges won't negatively impact your flooded chassis batteries if you have AGM house batteries.
AGM batteries have a number of benefits over flooded. The downside is that they cost more so it's always a decision as to whether yopur particular usage justifies the extra expense. In the case of engine starting batteries, that's pretty hard to justify. But in the house section, that's another story.
As previously mentioned, AGMs charge faster due to less internal resistance. That can be a benefit in certain situations. They don't outgas like flooded batteries do so the terminals and connections don't get the corrosion that you get from the sulfuric acid vapors present in flooded batteries nor do they have the venting requirements of flooded batteries because their hydrogen output is minuscule at best. That gives you a wider choice of where to locate them.
Batteries also have a recommendation to never discharge below 50% or else it takes a toll on their longevity. In reality, it's not the 50% that's the cutoff point. It's the voltage drop at 11.8 volts. On a flooded battery this happens to come in around the 50% mark, which is why that figure is often mentioned. But an AGM has a flatter curve when comparing its discharge on a voltage versus amp-hrs used graph. An AGM battery will run a bit longer, allowing you to use as much as 30% more amp-hrs from the batteries until you get to that 11.8 volts. That really gives you 30% more amp-hrs, which is like getting a free battery on a four battery system.
As to longevity, that will vary according to quality. I suggest getting good batteries, rather than price if you want to go with AGMs. The Lifeline batteries are the best. They will give you the 220 AH that your flooded batteries have (keep in mind that you can only use about 110 AH from your flooded but more like 140 on the AGMs) and are designed to last longer than flooded batteries, although that always depends on the way they are treated.
Your Magnum inverter recognizes the difference between Lifeline and the other brands and will have settings available that let you choose the Lifeline AGM batteries or other AGM batteries as well as flooded batteries. I believe the choices are AGM-1, AGM-2 and Flooded. I don't recall which AGM is for the Concord Lifelines but your Magnum manual will show you.
Can you justify the upcharge for AGMs? That depends. I have eight of them in our coach and generally don't boondock but when I do they give me plenty of power to run the residential fridge and other stuff, especially on a cold night when the Aquahot's diesel burner is running non-stop. I don't have any corrosion issues on my cables either. But eight of them do make a dent on the checkbook. My brother-in-law had two 6 volt batteries in their coach and they needed replacement. He considered AGMs but they are plugged into campsite power 99% of the time (fulltimers) and he didn't have that bad of a corrosion issue. Corrosion comes from outgassing, which comes when the battery is charged with excessive voltage, most notably on the float cycle. If your inverter-charger is doing its job you shouldn't have issues with having to add water all the time or cleaning terminals. So in his case we opted to just buy a pair of flooded batteries.