There really is a lot of personal preference, both human & equine, involved, as well as what is available to you & what you are comfortable managing.
My personal preferences based on the last ~15 years:
-Straight loads for the big guys. My horses are 16+ hands, funnily the smallest at 16.0 is the longest, he does not fit at all in a standard slant stall. The few times he’s had to ride in a slant load (when we traveled with someone else), he went in the back stall & he always tried to wedge himself as close to straight as possible so he could balance on his butt. I am also big on being able to unload any horse at any time in any situation without having to move others. I do like open stock trailers too, very versatile.
-No solid mangers. I had them in my first trailer, just dangerous & I hated that my horse couldn’t lower his head.
-Step up. I train mine to self-load on anything, but have seen too many go off ramp sides & hurt themselves or handlers, including horses I was helping train. I do spend time teaching all of mine to back off the step calmly with voice commands & consider it a very useful skill – I want to be able to maneuver my horse’s feet anywhere at any time & this ability is very useful on trails, in sticky emergency situations, or other tight spaces. Just like every other trained skill, I introduce it in logical steps. I also have some back & knee arthritis & one less heavy thing I have to bend over & lift is good – age gets to us all. I don’t think ramps are necessarily evil, I just don’t want one. It’s also usually cheaper without one.
-Even though my trailer is a 2H straight with step up, I train & expect my horses to load & unload from all types of trailers. If there were some emergency where they can’t ride on my trailer, I don’t want my horses to lose their lives simply because they wouldn’t get on a slightly different kind of box on wheels. In my personal opinion, this is the #1 most important aspect of a horse owner’s approach to trailering. Sometimes horses come with pre-existing baggage, I recognize that, but it’s to your advantage to try to have your horse as versatile as possible – doing that work can truly save his life in the future.
I LOVE my trailer, it’s a simple steel Adam BP with stock sides & a dressing room. It’s sturdy, well-built, airy & horses like it (they’ve fallen asleep in it, LOL). Divider is fully removable & a full escape door in front. 7’ tall has been plenty for my uphill-built 16.2 h TBs, but I do haul them with head bumpers just in case. The only thing I would change if I had money is I’d like a 2nd escape door on the other side – if trailer were rolled onto the right side & rear door was jammed, it would give me another way in/out. I prefer steel because it’s easier to work on, any decent welding shop can service it, maintenance is very easy, I can knock rust off with a little sandpaper & protect with clearcoat. I expect to have this trailer for 20+ years (I can’t believe I’ve already had it 12 yrs, it still looks great, stored outside) easily.
Both aluminum & steel corrode, maintenance is key to whatever equipment you end up with. Make sure its engineered for strength & stability, the lightest trailer may not be your friend. Stay on top of it, take care of your bearings & axles. Put good load-rated tires on it, take care of them (I do park my trailer on pavers & keep tires covered).