The old fella has to be first choice. Better speed, much better blocking.
You'll probably need to use the younger chap on occasion anyway due to the low endurance on the older player. Not to mention the 4 level gap between your health centre and training centre, that's just asking for trouble with a player with endurance that low.
Keep the younger player on the bench, but also set him as one of the overage players you can use in the u21 side to maximise the xp he'll get during the season, and give the young player a speed boost.
Unless you find a better keeper on the market or have another decent youngster ready to break through.