Map Availability
Maps are much more readily available with laptops. A full computer supports multiple applications, whether you use a proprietary software that comes with your GPS or if you use a generic GPS/Mapping application such as Fugawi or OziExplorer. With both of those peices of software, you can import any digitized map, scanned or otherwise, and calibrate it to georeference with a GPS. See our related article on Calibrating Maps for OziExplorer.
With a handheld unit you are limited to the maps provided by the GPS company, either stored with internal memory or by a flash style memory card. In modern countries the map availability and coverage is excellent. In developing countries it can be hit and miss.
Reliability and Durability
It took 6 months of overland travel for us to go through one standard laptop. It was an old Dell PII, and just barely ran Windows 2000. But for mapping it was all we needed and did the job. But all the banging and shaking in the truck broke the screen hinges. Dirt got under the keys and the CD ROM stopped working. Screens on standard laptops are hard to read in the daylight as well. We've upgraded to a Panasonic Toughbook now - vibration, dust, water, and actually reading the screen in the daytime are no longer issues.
On the other hand, a handheld GPS is often water resistant, and can stand being thrown around, for a fraction of the price of a laptop.
Cables, Cables, and Cables
I hate the cable between the gps reciever and the laptop. I have similar disdain for the 12V power cable for either machine. I've since converted my serial gps to a bluetooth unit to eliminate the one cable between computer and GPS. A docking station mounted onto a laptop stand is a great way to control the cables, but takes a lot of room in the vehicle.
On the other hand, a handheld GPS doesn't need to have any cables as long as there is battery power. Eventually you need to plug into 12V DC. Except for a more permanent gps unit mounted into a dash, cables are all but unavoidable.
Other Perks
Laptops provide the flexibility of having offline email and the ability to store digital photos and movies without running out of flash card space. We like to share music with other travellers via thumb drives and CDRWs. We also maintain the ability to maintain a website while on the road. Handheld GPS units do none of these things.
I like the big screen of a laptop. Route planning with a handheld is next to impossible.
Summary
If you're staying close to home, don't need access to computer applications, storage, or you can rely on the maps provided by a GPS company a handheld gps is simpler, cheaper, and more reliable. If you're planning an extended international trip, you probably want the flexibility and power of a full laptop.
I’ve always been fascinated to read the reasons why other people used laptops (just like I do) rather than a small portable device. I like your article, and I linked to it from http://www.laptopgpsworld.com/general-discussions/202-laptop-gps-combination-versus-handheld-gps.html