All depends on what Bruce Arena feels like doing with the national team. Olympics fall in the middle of the first group of World Cup Qualifiers.
I would guess Howard or Keller, my pick would be Keller. This would allow Howard and Friedel to train/battle it out with the full national squad. Would also "reward" Keller for a great career for the US as he'll be old by the time the WC rolls around. It would also allow him to impart some of his wisdom to the actual U-23 keepers - Countess, Warren, or maybe Cronin.
PS - Not really familiar with the EPL schedule, but I assume they're on summer break at this point and if ManU come touring the US again, Howard would be a big draw.