I think that a standing stall would be fine if the horse had atleast 3-5 hours of pasture time every day, as well as good 4+ workout rides every week. Otherwise, I really don't like them. It's always best for a horse to be able to move, in order to keep healthy. It helps them digest their food and keep good circulation in their bodies. If they're going to be left in their stall almost 24-7, then it's just not healthy for them. A standing stall should also be picked atleast 2 times every day and be well ventilated, IMO.
We recently brought in four OTTBs and two sucklings from the Sam Houston Racetrack. All six horses were crammed into three nearly standing sized stalls (10 by 10 I believe?), incapable of moving more than a few inches. Two have severe thrush, all of their muscles are atrophied, and one has lung problems due to the ammonia produced by all of them. It's sort of a 'worst case scenario' but it does show that bad ventilation and no chance to stretch out and do what horses do can ruin a horse.
So, good exercise, access to pasture, and good ventilations and I'm good with a standing stall. Otherwise, I'd say just put in the extra money for a box stall.