If it were my horse, I would pull & band because I like that clean and put-together look. I only like long manes if they are in perfect condition, with equal length & thickness throughout. Henny hasn't had the years to get there yet. That said, Henny will be his adorable, personable self, regardless of what you opt to do with his mane! A pretty color ribbon to match/coordinate with you could be sweet.
I love his mane, so beautiful!! I think you should leave it down. My second choice would be the running braid.
Or maybe you could have the photographer take photos of both ways, if you both agree to experiment!!!
Haha I ended up braiding his hair the day before and clipping his bridle path. When I took the braids out, his hair was sooo pretty and wavy! Don't have pictures of it yet, waiting on proofs from the photographer and pics from my friend. :)