Here are some things to consider... 

Loading: Horses don't necessarily load more easily into a slant -- they will load very easily into an open, airy, light trailer of ANY kind. Horses are generally quite happy to load into a Thoroughbred-style front-door trailer: the handler can lead the horse in and then duck under the chest bar. Horses will also load more easily when they have been taught to load correctly - and when they aren't anticipating a rough ride, which has much more to do with the driver than with the trailer design.

Unloading: It's not possible to lead horses out of all -- or even most -- slant-load trailers! If you have a large three-horse slant-load trailer and small horses, you can generally lead the last horse out, but the first two will need to back out unless they are truly small and short-backed. Thoroughbreds and other long-backed breeds will almost always need to back out.

Dropping heads: It's very true that horses travel much better when they can drop their heads, both for muscle comfort and because their respiratory systems can function MUCH better that way. But again, unless you have small, short-backed horses in a slant-load, they won't be able to do this even if you tie them long. There just isn't enough room in most slant-load trailers -- the stall length is measured diagonally, and horses don't stand diagonally! If you measure the stall length from nose to tail, there is usually significantly less space available in a slant-load -- sometimes several feet less -- than there is in a comparably-sized straight load. Again, the horse would be better off either loose in a stock trailer (or in half of a stock trailer) or in a straight-load Thoroughbred-style trailer with only a chest bar in front, and a LOT of room in front of the chest bar to accomodate the horse's choice of head/neck positions during the trip.

The dividers in a slant-load are similar to those in a straight-load, and in either case, a padded partial divider is vastly preferable to a floor-length divider. Horses should be able to spread their feet to balance themselves, and a partial divider will allow them to do just that.

It's true that horses will naturally tend to place themselves at a slant; many will turn all the way around and face backward, which is probably the MOST natural way for them to travel in a trailer. Unfortunately, few trailers, and no towing vehicles, are designed to allow this! Horses like to stretch to take up whatever space is available, which is why it's always nice to remove the divider or tie it back if you are hauling ONE horse in a two-horse trailer of any kind. But whether they are expressing a strong preference for travelling at a slant, or whether they are simply standing on the diagonal because that allows them more freedom of movement, isn't entirely clear.

Some people perfer straight-load trailers; some prefer slant-loads. Either way, it's a good idea to learn as much as you can about BOTH types and all the variations available, before you make up your mind to buy one -- whatever you get is something you and your horses will probably be living with for a long, long time.

One other point...if you have Drafts...forget slants :-)

I like straight loads.

Another thing to consider is that you can load and unload in any order in a H2H trailer.  You can also in a pinch take out all the dividers and pack it full, like if you were being evacuated for example.  (Full of horses or other items.)