First off, you will not need a GPS on the Whitney trail. It's a nice thing to have anytime you're hiking, but a simple trail map is plenty for this heavily used highway. There will be about 200 other people on the trail with you. The following advice pertains to more challenging terrain, especially off-trail.
My advice as an engineer who reads contour maps professionally - use a paper map with a GPS. Two reasons 1) on-screen maps are too small to give you the big picture and can waste time scrolling around 2) If anything goes wrong with your GPS you are screwed (dead batteries, dropped, lost, soaked etc) if you put all your eggs in that one basket.
Most GPS units now have mapping capability, but I wouldn't pick a model based solely on that one feature, and especially not for one particular trail. I'd pick one that is small and lightweight, is user friendly, has good battery life, and then I'd go for one with a compass built-in. The compass will point you to the next waypoint which is really slick. For me, it's also very important to have the GPS unit integrate with my computer mapping software, but I'm probably a bit more into this than most people.
I upload my own routes to my GPS from my computer. I use Topo! software (National Geographic) to layout my route on a color contour map on my computer screen. I trace the route with my mouse and include waypoints at key locations along the way. I label it all nicely and then upload the route with all the waypoints to my GPS unit. Then I print the exact map or maps that I need with GPS coordinates as a grid on the sides of the map. I can also print a profile showing elevation and distance to gauge how steep the trail is.
At the trailhead, I tell the GPS to follow the route to my destination. It will identify the next waypoint and the compass arrow will point me in the correct direction with a bearing and distance to it (assuming you have a built-in compass feature). When I get to that waypoint, the unit automatically advances to the next waypoint of the route and a new compass bearing will direct me there. It will even calculate how long it will take to get there based on my rate of travel. A preloaded trail in your GPS should be able to do all this, but it's only good for the trails that are preloaded.
Modern technology is great, but there's no substitute for learning map and compass skills as the foundation for all this. The time invested will pay off as you hike further into the wilderness. At the very least, learn one critical skill very well - how to extract your coordinates from the GPS and then located your position on a map using the GPS grid. If you can do this, you'll always know where you are on a map. In reality, this one skill is all you actually need and any tiny GPS unit will do.
One last tip for advanced GPS use - learn how to use the tracking feature. This can be a life saver if you suddenly find yourself stuck in a white out and need to get off the mountain. If you call up the latest track and instruct the GPS to back track it as a route, it will point you right back where you came from, step by step.
All these skills take some time to learn and some practice to get efficient. It's especially fun off-trail and it can give you piece of mind in a bad situation.
