I use a HP iPAQ rx5710 PDA with built in bluetooth and GPS with my Roberts PDA version. This PDA is maybe a little more expensive than a PDA that have the minimum requirements to run Roberts PDA but I use the GPS function to log sightings on Cybertracker etc.
You can have a look on this page
for some PDA models which are recommended.
As to choosing between a laptop or PDA? They are two different pieces of equipment for different functions so you'll have to decide what the ultimate use of the device is going to be. If you want functionality such as full Office programs then go laptop but then you will not be able to carry it out in the bush in your shirt pocket
One word on Roberts PDA (or multimedia if going for the laptop) is that personally I do not like the drawings that much but then you also have colour photos that is very useful.
Just remember that you cannot buy the multimedia version for PC/laptop and then use that on a PDA and visa versa. The PDA version does link with your PC so that you can download your lists etc.
If going the PDA route I suggest you buy your Roberts PDA version on the largest SD card you can afford as the extra space on the card can then be used for other purposes/applications. I have the 2 Gig card.