On the Cyclocross. They ride well on rough trails and even offroad in the summer----Or winter if you are hardcore---far better than a road bike would. I am an EX mountain biker and I was thinking of a CX bike to supplement my road bikes. As a supplement it would serve a purpose but as my main Road bike it would not be the best choice.

The Specialised Roubaix is a favourite bike for quite a few and not just on 50+. Can be set up to give a more relaxed body position with the bars a bit higher to save the back if that is a problem. Must be others with the same geometry and others within the Specialised range that would also be suitable for you. Gearing is one thing to look at and you do sound as though you have some fitness about you. Compact/Standard double cranks or triple crank is the main option on gearing and the choice is down to how steep your hills are and how fit you are.

The best bet is still the LBS for advice but most of us will stress that finding the right LBS is the difficult part. They will offer you the options and advise you on them and that advice should be aimed at you and not what they have in stock and want to shift.