Two of the same kind. Optima Yellow(if you plan on deep-cycling them), Odyssey($$$), many say the Exide and other common brand AGMs are fine.
Sears Die Hard Platinum was the choice until recently, they used to be rebadged Odysseys but are now apparently changing to another mfg. They might still be fine.
I have an Odyssey 34R[r=reversed terminals]/PC1500 as the house batt, and the same size Die Hard Platinum as the guest batt. I'd make the guest(aux) batt larger(like Group 31, or, better: two group 31s) if I could do it over. My Engel 45 fridge will run down a group 34 in three(maybe 3.5) days in hot ambient temps, but run longer in winter.
I'd like to able to go 6 days in summer.