If the final destination is use as a round pen, go for the 6ft height. A horse who is crowded during round penning is "less" likely to go over a higher fence. I have seen horses go over short panels, doesn't take much crowding, pressure by you or another horse in barn situation, they are just too short. Some animals would go over a 7ft panel too, arguing with you.
Six foot high panels are still going to be challenged now and again, by a horse feeling pressured especially if this is a commercial barn, has lots of horses in for training. You get all kinds. But height is a helpful vision aid, so animals are better at being redirected sideways, not over. Have to say many of the "western horse" trainers are dealing with slightly smaller height animals. A lot of lines of QH are small because that size suits the work they do, reining, cutters, stock horse. Get a horse up over 15H, they can't turn as well or as fast. Roundpenning a TB of 16H, his head and neck are LOTS higher in the air, just standing still.
One of our 16H 2yr olds, just climbed up and over our 6ft wooden fence round pen walls. He did not understand that he should go sideways to get handler pressure off, just went over instead. Like a kid scaling a ladder!! Did not get hurt, pulled off one board 2"x12", but made us RETHINK how he was being worked. He didn't understand, we had not done our part well. Backed up, fixed his holes, then went back to forward learning progress. He has never offered to climb out again. We use the pen now and again for a couple specific things, but not often. I do prefer our wood fence pen over steel panels, but it sure is not portable!
Mine are about 5'x12', with a 4' gate panel. They are the kind that have the chains instead of pins to hold them together, makes them a bit more flexible(edges don't have to line up exactly) and nothing to get lost. Also they have the square/flat tops, which are safer because there isn't a hoof-sized space at the top for legs to get stuck in if somebody rears and comes down on top of the panels, as mine like to do. I got my panels from a listing on Craigslist, and they were CHEAP and hardly used, so I basically was going to bring home whatever was there unless it was a pile of junk lol. They are similar to this: http://www.tractorsupply.com/livesto...tility-3602990