It has a lot to do with what you plan on doing with the trailer. We used to have a small 2 horse that was open in the back and sides. Many of our club organized rides are on someones ranch down 20 miles of dirt roads. Open trailers are fine if you don't mind getting to where your going and having to worry about whether your horse has inhaled to much dust and then clean the layer of dust off the horses and saddles.
The trailer we have now is enclosed. I much prefer that. If you plan on traveling on mostly paved roads, then a stock/open trailer is going to be much lighter weight and less costly.
We have a small Sundowner that we have been very happy with. The interior is very horse health friendly. I like lots of padding around the horses on the walls and dividers and Sundowner is great at providing them.
This is a nice entry level horse trailer that looks like its in really good shape. The price is right too http://www.horsetrailerworld.com/hom....asp?ID=237206