I have owned both.
The slant I could get more horses in, as in I had no trouble getting them in or out. You can turn horses around instead of backing them off. But that also depends on how large or small your trailer is. Another thing about slants is sometimes larger, longer horses can't fit comfortably. We always had to put the long one on last because then he had more room.
The straight can be a little more tricky. Mainly because your horse must go in before you (if you are alone) while you are outside waiting for your horse to stand and stay in the trailer without backing out. And you must never tie before you put the butt bar up first. Most newer straights are taller and are more roomy and can take the longer horses. I had to train my horses how to load in this trailer and it took about a month.
Those are my experiences with the two. My next trailer will be a stock combo.