Marty,
Very doubtful that you'll get 8.5 years out of lead acid batteries. However, 5 years is very common if you take care of them and keep the water level up. Based on your stated usage lead acid should work just fine for you. AGMs are nice but pricey. I could never make the economics work out as they cost about 2X lead acid but don't normally get 2X the life. However, if you don't want to have to check your water level and top off lead acids about every month, then AGMS are probably the ticket.
I've not had any experience with Costco's batteries. IMHO you can't beat Trojan 6V golf cart batteries. The T105 model is what you want. Check around for pricing. I think I paid about $110 ea for mine. BTW, if you switch to lead acid, be sure to check to see that your inverter/charger is set for lead acid and not AGMs (not sure if older inverters had this setting option).
Steve