Shape, size, surface, slope, surroundings. Rule 5 doesn't feature in my decision-making as it won't when one happens for real. And I always choose one that I could get into if I had to, and treat every PFL as if I might have to use the field should the engine not respond for the go-around.
On the Chipmunks I instruct in this is a not uncommon occurrence (despite mentioning it during the briefing) as the front-seater can catch the mixture with the cuff of his nice flying gloves, advancing both throttle and mixture lever together for the go-around (on the Chipmunk forward mixture=lean mixture!)