Do they have models which have colorado lakes charted and mapped with underwater features and depths? That would be cool. Easy to get directly on top of underwater features without having to find key spots on the map, locate the spot with your sonar, then mark it with the GPS. My GPS is the basic, basic model and I haven't really done any research into the lastest and greatest.
I have the Lowrance iFinder H20 and I really like it. I also have a Lowrance unit on the boat, so I can transfer the saved waypoints back and forth using MMC cards. That's a huge benefit for me. Structure I mark on my boat mounted unit I can quickly transfer to the handheld unit for ice fishing and vice versa. Lowrance has a few free bottom contour maps for Colorado and Wyoming lakes, too, including some that I fish a lot (http://www.lowrance.com/Mapping/LakeMaps/index.asp). The Lowrance units accept the Navionics maps and charts, too.
It largely depends on personal preference. I've used Lowrance sonars with good results for years, so when it came time to get a combo sonar/GPS unit for the boat that made the choice pretty easy for me. And it didn't make sense to get handheld that was incompatible with what I had on the boat. So both are Lowrance.
I just bought a Garmin GPS 60c map. It is pretty nice since it has color and you can store up 56 mg of memory. I use the GPS to mark fishing spots and I can also use it for for driving around town. You can upload topography maps and fishing maps with fishing hotspots. The only downside is that you have to use Garmin mapping products which can get pretty pricey.