The more that you can outline your intentions for a GPS receiver, the better you will do in the selection process.
As others have stated, you do not necessarily want a touch-screen product, because they tend to use a lot more battery power. Battery power is a pretty big item for a backpacker, because spare batteries weigh a lot.
I've never carried spare batteries for my most current receivers (dedicated GPS receivers, not smart phones) because I know that they will run for a day of continuous use. I never run my GPS receivers continuously. I will turn one on at least once per day, or maybe several times if I think navigation is getting dicey. That works well with a large paper map. There is tremendous viewability of a large paper map as compared to the tiny screen on a GPS receiver.
Personally, I am not a big fan of barometric altimeters. Either you are trying to calibrate it off GPS, which has errors, or you are trying to calibrate GPS off the barometer, which has errors.
At present, Garmin is the market leader, at least for backpacker consumers. Virtually any Garmin product that has been introduced in the last five years is probably fairly good. However, again, you have to decide how you intend to use it. For example, if you intend to use it in your car, and then dismount and use it on the trail, that means something. Many GPS receivers will work fine on their internal antenna even if you put them in a car, but you may have to put it far forward on the dashboard for the antenna to get a good view of the sky. That location may not be good for your vision, so you may need to place it closer to the driver. Then when it gets that far underneath the metal roof, the antenna no longer has a good view of the sky. As a result, I like to use my receiver in my car, but I have an external antenna placed forward onto the dashboard. Besides, in a car, you are probably using the car's +12VDC power, so you have a power cord hooked in beside the antenna.
Typically with many modern GPS receivers, there is a base map that underlies the data screens. Do you want a base map of topography, or streets and roads? Hint: many backpacker-type GPS receivers come out of the box with a base map installed, but it is a wrong-scale map that really isn't much good for detailed navigation through the woods. All it really does is to inspire you to purchase some real base maps at the right scale. With other receivers, you buy a map database on a memory chip, and you load that. With others, you buy a map database on a DVD and load it by way of your computer and a cable. With some GPS smart phones, you are trying to connect to some map database service, so you end up waiting and waiting, or else you pay through the nose for cellular data minutes.
Most GPS receivers are made to be durable. They will withstand a lot of abuse unless you are a real klutz.
One model that is my favorite now is 16 years old, so it isn't complete junk.
Lots of GPS receivers sell for about $200-250 these days.