I've used both. We used to use a laptop and a Garmin eTrex. I had the Delorme software and it worked great. You could avoid sections of highway if you knew there was road construction or an accident ahead. It was a pain because of the wires, but not too bad.
I now have a Garmin Nuvi and it is great too. It does allow you to reroute, but you cannot specify what section of the trip you want to route around. So, in some ways it is not as complete of a system. But it certainly is easier. It also is more expensive.
If you go the laptop route, you will need an inverter to power your laptop charger on anything except a short trip. We always used a cigarette lighter plug splitter that we could run the laptop charger and the gps charger off of. Garmin has a dual gps cord that allows you to power the gps from the cigarette lighter and plug into the port on the laptop.