The big advantage of a laptop is the type of mapping software you can use - I use Maptech Topo software on mine, which is way cool and very detailed and accurate - this level of detail would never fit on a hand held (does export to a PDA though!). Plus I can do 3-D views, routes, profiles, etc, etc, using the full power of a laptop.. Down size is the size and weight, and power requirments.. I have an AC adapter for my truck, and then dual batteries which would give me about 6 hours real world usage. I can connect it to any of my GPS's, including my Bluetooth wireless one.
Next is a PDA, I use an IPAQ HX4700, it will run mini versions of most of the popular software, including Maptech.. It's much smaller and easier to cary around, but not as easy to work on and use, or to view as a laptop. For these I'd only pretty much get a unit that has bluetooth, and use a bluetooth GPS with it. The unit is a bit more fragil than a true GPS unit, you might not want to be dropping it or stuff out on the trails.
Then there is the GPS units themselves, they are the smallest and easiest to carry around (car mounted units not included) , but then can be limited on map detail.. I have a Garmin GPSMap 60cs, Garmin has some topo software that can be loaded onto it but it's not as detailed as the full laptop, and it's limited on how much it can hold, but does pretty good for a handheld pocketable unit..
In the end, I find I use the laptop for big trips and trip planning, or trying to find hard to find spots in the field where I need real detailed live updates on where I'm at and going.. the PDA make a nice toy but does neither well, and for everything else I use the GPS unit.