2 60s will be easier to handle, and 20% more watts.
The factory alternator to battery charging wire won't be either #2 or #4, so why bother going larger than that wire if both batteries are under the hood? The only reason to do so, is if you ever plan to jump start off the aux battery or tie them to run a winch - which a diode isolator won't do anyway.
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker