The dilemma is this really
Ruddy is the number 1 GK, no doubt. However he has not played a Premier League game for what 6 months now? So he will be rusty one would think.
Bunn is an ample backup. Has performed well, and is the current holder of the shirt.
I think either will do a fine job. It's just a very tough choice because Bunn has played pretty well in Ruddy's absence