If it were me I'd go for the bigger size. Just think of the extra footage as less place for them poop and roll right into it after that lovely bathing/grooming that took ALL day to give them. Plus it would give them an actual bit of walking room rather than just turning around room.
We had a gelding who would weave in anything smaller than 12X14 ft. As long as he had movement room he was great. If not he'd weave himself into a tizzy and be all het up 24/7.
"Light travels faster than sound. That's why some people appear bright until you hear them speak."
Larry the Cable Guy