The chemical reactions responsible for beer 'mellowing', loss of hop flavor/aroma/bitterness, and beer staling happen faster at warm temperatures. The physical process of fine particulates settling out of the beer happen best at cold temperatures. Any of those temperatures would work, it will just favor one or the other. I like to do a period of warm conditioning (usually in the fermenter) before packaging and lagering, but that's just me. Once you like the flavor you should keep it as cold as you can if you plan to keep it around for a very long time. Before I got into kegging I would just leave all the beer in the 65-75° closet, and some of them started to show evidence of staling (slight cardboard flavor) at around the 6 month mark.