Reading this post, I was wondering when Elite was gonna be mentioned. I'm in the process of possibly ordering one myself. Coming in with a machining/fabricating background, I was completely impressed with fit/finish/function (3 F's) of the Elite. Integrity is another well built trailer in my opinion, and I'm getting quotes from them too.
I closed my machine shop down in Feb of '05 and have been hauling horses all over the US ever since. I started out by using my slant load Exiss (already had it and a dually Cummins) and can say for sure to stay away from the bolted together production type trailers. Custom built, welded together trailers are the way to go. If the rear doors, side escape doors, gates etc are bolted on, walk away. Look at one of these trailers and then a welded trailer, it's quite obvious. Also look at the floor thickness and spacing of the crossmembers. Have you ever seen a horse go through the floor while in transit? It's not pretty, trust me.
I personally wouldn't even consider a steel trailer. Outside of the weight, it is eventually gonna rust. I've had a couple, and still have a small one I keep for day rides and trips to the vet.
Another thing to consider is a box stall trailer. I've got a slant with weekender, but going to a stock/combo with weekender. I've been using my friends 3 box stock trailer lately and like it way better. A 2 box stall will get you 4 horse capacity.
The weekender allows me to camp with my horses, and cut down on motels while hauling others horses. The box stalls allow the horses to stand however they wish, which is usually backwards. If you are gonna be traveling great distances like I do, the horses come off fresher looking in the stock type/box stall trailer than they do in a slant. In my business, a horse has to look like it went around the block after traveling from Va to Ca, in your case, you want your horses as comfortable as possible ... ... ... ... . and I think the box stall does it.
Another thing to consider is axle location. (Elite will put mine where I want them) The stock type trailer I've been using is a Travalong, I'd never really heard of them and it seems like a decent trailer, but I don't like the axle placement. This trailer is a couple feet longer on the floor than my Exiss, but rides and manuevers like it's 10 feet longer because the axles are all the way out in the 3rd stall.
One more thing, I didn't look, but if your hauling with a 4X4, make sure the axles are blocked. If not, you'll be replacing rear axle tires, tailgates or both
