The answer is “it depends”. What are you going to be using it for? What kind of horses are you going to be hauling?
For example, when I’m showing, I show big hunt seat horses all around the southern US. Primarily Texas, but we have a bunch of shows in Oklahoma and surrounding states. I prefer a big, slant load gooseneck that has the height and width for my large horses, but I prefer the stock-esque style build over the completely enclosed style since it has better airflow and it’s usually hot where I travel. The gooseneck is better stability for multi-hour hauls and highway speeds.
When I’m not showing and generally just using my trailer to run around to vet appointments, or moving a mare/foal pair, or little local events I prefer my little bumper pull. I have a few smaller western horses that fit very well in it and it’s not a pain in the butt to bop around in. It’s a three horse slant, and I can fold the slants back to fit the bigger horses when needed. Better gas mileage, easier to maneuver.
I prefer slants over straight loads, but mostly because I have a lot of horses. If I only had 1 or 2, I’d be very interested in a 2+1. They aren’t popular in Texas, though, so they tend to be pricey. A friend of mine has a four horse gooseneck with four straight stalls and a mid loading ramp that I would also consider using, but for the length of that trailer, I could get a 6-7 horse slant.
The best recommendation I’ve ever had is - if you’re hauling a lot, go ahead and try to buy new. That way everything is under warranty. If you are buying used, make sure someone knowledgeable gives the trailer a once over - especially with gooseneck trailers in the last couple of decades, we’ve seen a lot of bad welds that crack and cause major issues. If they’re caught, they’re fixable - but if they aren’t it can cause disaster.