Depends on what you want it for. I think stock trailers are great for just getting places, they have their draw backs, tho. Being open, your horse is more prone to get injured from road debris or getting injured in the panels if something happens (I've never personally had issues, but I have heard others have issues). Horses that have issues trailering will usually load better in stock trailers because of their openness. They are also more versatile in their uses. But, don't generally offer weather protection or an area for tack. Most of the cowboys/ranchers I know use stock trailers for both horses and cows on a regular basis.
I would mostly focus on what you are going to be pulling it with. Most of the stock trailers I have come across are deceptively heavy, in both tongue weight and total weight. If you have a regular mid sized truck, you would be better off getting a lighter weight horse trailer, even if it costs a bit more. Around here, you can find a decent 2 horse straight load for a couple thousand. Stock trailers of the same length can be twice as heavy (but there are those that aren't very heavy, either, just need to pay attention). Tongue weight isn't usually measured (it is probably more important than trailer weight, though), but as a general rule, you don't want your rear wheel wells going down more than 1/3 of the regular distance between your wheel well and tire (fully loaded, horse weight and all).