As long as it has a temperature gauge, I think that's the only "must have". Some of the differences are;
1. Where the fan is located. They are either on the top, bottom or back. I've read that it's best to get one where the fan is on the back because it dries all the racks more evenly. I have one where the fan is on the back, and I don't know that it makes any difference. I would recommend one that has the fan on the bottom or back though as opposed to having the fan on the top/lid. Only because it's kinda a pain (and a bit heavy) when you are constantly lifting/removing the top to add cookies.
2. Stackable trays or sliding trays. Sliding trays are much easier to use. It's much more convenient to slide a tray open and put a cookie in, rather than having to lift the trays and restack them to close. This is just a convenience factor.
3. Circle/Square vs Rectangle. I believe (though I could be wrong) that all square or circle dehydrators have a center "pillar" to allow better air circulation and to be able to stack the trays. The problem with this is that the trays are very slightly angle down towards the center. It's only a VERY slight angle - but I was having the problem of the icing run off the side of the cookie when I put them in the dehydrator. I found I had to make my flood icing a bit thicker to avoid this problem. I was also having the problem of the icing drying with cracks because the icing was shifting during the drying process because of the angle of the trays. This could be just my problem with the consistency of my flood - I'm assuming not everyone has this problem. The other issue with the center "pillar" is that you can't fit as many cookies per tray like a flat, rectangle tray.
3. Size. Make sure to note the dimensions of the dehydrator! Some of them are BIG!
4. Price. Excalibur Dehydrators are really expensive but are considered the "best". If you are using the dehydrator for only cookies - I don't think it really matters. I don't think name brand matters here.
I have a LEM dehydrator and love it: http://www.lemproducts.com/pro...4177/food-dehydrator
I got mine for $100 when it was on sale.
If you're a hobby cookier, and only make a couple dozen cookies here and there - go to Walmart/Target and get a circle one for $40. It will do what you need it to do and won't break the bank. But if you're making several dozen cookies a week - I highly recommend getting a rectangle one with sliding trays. If you don't, you'll get frustated with the stackable trays and eventually go out and get a different one in a month or so (can you tell I'm speaking from experience?!).