We've got a 4 horse head to head Sundowner, so 2 face forward, 2 face backwards. We looked at lots of brands and lots of configurations. I did quite a bit of research before we ended up making our decision and we've been extremely happy with our purchase. Here's what I've found:
1. A slant load trailer is typically built for horses 15.3 and shorter. In order to give the horse ample room to move their head to clear their lungs and breathe properly, they need the extra space of a straight load. (Our 2 year old is 16.1... he should mature to 17.1ish we think. Our 7 year old is 16.3. We also move other peoples' horses as well, both for shows as well as transport around town or within the SouthEast.)
2. It doesn't matter that a horse wants to stand at a slant when they're in the trailer. It's physically healthier for them to not have to use diagonally opposite legs to try and balance on. Think about it... when a horse jumps, they take off and land on pairs of legs. They're used to using their legs either both fronts or both rears for most balance situations. The same goes for trailering. I've seen horses that like to stand in the mud as well... is it good for their feet? No. Do we let them continue to stand there? No.
Just about every horse has gotten on our trailer with very little to no hesitation. We've got a rear ramp and side ramp, though we typically load and unload everyone through the side ramp.
Anyhow, just my two cents... your results may vary.