With today's bow 60lbs takes care of anything on the continent. Go for the lighter bow. Your neck will tank you for it as will your whole body as you age. You now have the option of backing the weight down if need be. Make it easy on yourself.

Fieldmouse, Lenny is right that a bow performs better at the higher end of it's range. It'll be a little faster and quieter for a given poundage. However, a bow backed down will still perform. It just won't do it with the same efficiency.

From what you say of your injury I would go with the 50-60, that will allow you to back it off if needed, and Len knows his stuff (Well that is what David says[8D]).

I think you did the right thing as well. I have been shooting 70 pound peak weight bows for years but recently picked up a 60 lb model that peaks at 62 lbs. Not as much KE as I would like but still very respectable speed numbers with some of my setups. I would definitely take it whitetail hunting.

If you're not comfortable shooting much over 60 pounds... that's your answer. Get a 50-60 and peak it out. If he has another bow of the same make/model try it too. One may peak at 59... the next at 62or 63.
Me... I prefer closer to 70.. I dropped down 3 years ago from a 70-80 to a 60-70 and never looked back. I still shoot the 70-80 set at 76... but I prefer my lower bow set at 67 or 68.