Elena, let me throw out some suggestions before you purchase a receiver. First, have a very good idea what you intend to use it for. To be specific, if you intend to use it purely for trail navigation, that means something. If you intend to carry paper maps along with it, that means something. If you intend for all maps to be in electronic memory of the unit, that means something (but the tiny display can't show you much). If you also intend to use it in your auto for road navigation, that means something. If you use it on a trail, do you intend to keep it running all the time, or would you use it just a few times a day or whenever you need it?
In general, the brighter the screen display is, the more battery power it will be using. Some will "time-out" from bright screen and go to a dim screen. Some always run in dim screen mode when on internal battery power, but they will run with bright screen if you are using external +12 from your auto.
Some map products can be loaded into its memory, but some of the maps are really good, and really expensive. Others are almost worthless for trail navigation, despite their marketing.
Maybe I am old-school here, but I plan my wilderness trip at home on the computer using TOPO!, and I will edit in the waypoints or mile posts that are important. Then I transfer all of that data to the receiver. I print out my customized TOPO! map in color, store it in my map case, and carry that inside my shirt while out on the trail. That's another way of saying that I put it somewhere safe. If weather is bad, like on a snowy X-C ski trip, I can run the receiver to point me from point to point. If things get messy, I can pull out the paper map and use a compass.