I'm in a quandary here. While, like most of us, I would love to shoot a big racked mature buck, I do know that a 6 1/2 yr. old buck is going to be a bit chewy. Age is only one component of what it takes to produce a large rack, but it is the one that may most effect the taste of the venison. In the situation described in the original question, I would shoot the younger of the two, if the shot presented itself. If the older buck had a much bigger rack than the younger one, I certainly would be tempted to take that one, but then I would have lots of tough meat to deal with. Not completely sure what I would do in the heat of that decision.
Keep life simple...if you hear hoofbeats, think horses, not zebras.