I prefer straight load because I find it gives big horses WAY more room standing than the slant load trailers. I find it is easier to load (as long as you have an open front) and find it safer to unloaded. Ones with a ramp are ideal.
The pain different is that with a slant load you can fit more horses (hence they are in a slant) but they have less room. I find it also difficult to turn them around inside before unloading.
The only good thing I personaly like about slan loads is that the horses are balanced right in the middle of the trailer as opposed to just one side if you are trailering one horse in a straightload.