RE: which 300?
Of all the magnums I'd probably select the .300 Ultra Mag for no other reasons than the non-belted case and the ability to handle the biggest bullets without serious reductions in performance. Keep the weight of the gun over 8lbs total and put a good pad on it and it'd be manageble with practice. If you want to shoot it a lot, put on a removable brake. It'd also give you a great reason to start handloading if you don't already.
Mike