The area where I have seen the biggest difference is them charging when we are on trips and anchoring/mooring for two weeks at a time. With a 660 AH bank and a 75-80 AH per diet I have been able to go 4 plus days without turning on our engine. With our 80 amp alternator it takes less than 3 hours to bring us back to 85% capacity. My friends must run their engines 5+ hours with the same configuration to acheive the same results. (BTW this is the hidden cost of the wet cells..acceptance rate and the cost of recharging both in alternator or shore charger use can be twice as much)...............
This is interesting and there's certainly something to what you say. However, the figures cited above are misleading.
First, with a 660AH AGM capacity, you're WAY WAY underpowered in the charging dept with only a 80A alternator. That's only 12% of the AH capacity of the battery bank (80/660 = 0.12). And, in the real world, no 80A alternator is going to put out a full 80A for hours on end, and with your 12% of AH capacity alternator those AGMs would accept a full 80A output for upwards of 3 hours or more.
If you had a larger alternator, say 150A, you could reach that 85% SOC target much, much faster, thereby realizing additional savings in engine run time.
Similarly, your friends who have the "same configuration" with flooded batteries are also under-powered in the charging dept. Those 675A flooded batteries (assuming Trojan T-105s) can easily take more than 150A charging current when 50% depleted. That's the size/type/make I have aboard my boat and, alas, I only have a 110A Balmar alternator and an AC generator which can power my Victron MultiPlus with its 120A charger. Wish I had more. However, even with this configuration I rarely need to run my charging sources for more than 2-3 hours per day, and my electrical diet is much larger than yours...on the order of 125-160AH per day.
Of course, many of us just go with what we've got until we are able to make intelligent upgrades, but the cost comparison between AGMs and flooded batteries is very complicated and I don't think we have all the empirical data needed to make a valid comparison.
Yet. I'm working on it, though, and already have in hand some detailed data from a major battery manufacturer, and hope soon to have more.
Meanwhile, don't sweat it too much. Get out there and go sailing!