No matter what....Alway's purchase a battery with at least 875 cold cranking amps...Interstate batteries are the best. Get the most CCA's you can buy and stuff in your car or trucks.
I just replaced the original Motorcraft battery that came in my 2000 Ranger. It was still working, but was weak after 6 years. I replaced it with a Duralast Gold from AutoZone. These seem to work pretty well.
A long time friend - mechanic of mine told me just about the same thing you said above. He got me into the habit of fitting the largest battery my battery holder can accept.