You want to consider the horse in work, not only on the more flat surface of a Dressage Arena, and often flatter area of a Cones course. In those places it is EASY for horse to go collected, neck more upright, because moving his vehicle and load is EASY on those good surfaces.
Change over to Marathon, which have hills, dips, gulches, cut areas, that require horse to do hard uphill work. Uphill work REQUIRES horse to lower head and neck to gain better balance to move his load over that kind of course. So a horse who normally goes up-headed, could need to get head, nose/muzzle down to his knees to keep his forward momentum.
So you might want to see if windpipe would be cutting into the V-type breastcollar edge, should animal lower his head “that much” on a Marathon.
My friend with his Arab cross does have one of the Super V type breastcollars, STILL has not decided if it really helps that much. He uses it because he paid a lot for it, horse goes comfortably in it, but undecided if the Super V is an advantage to this animal.
We have regular V-type breastcollars on our Marathon harness and like them. Horses like them, go well in them, though they are not the Comfy-fit brand. I know our horses do lower their head often on Marathon, so the V-cut is helpful.
I would like to try the Super-V, but fitting our horses is kind of pricy, so not likely to happen for them. Just would be interesting to see if they went differently.
You could order the harness with BOTH type breastcollars, use the V for Cones, Dressage, Pleasure showing. Get the Super-V breastcollar out for hard work days of conditioning and use on Marathon. With buckle-in traces, you can just shift them back and forth between the styles. Another option is to get the V-style, then save up for the Super-V, buy it later if you decide you want it.