If I had to choose between both formats at this exact moment I would go Laptop.
I used to use a D8 and now just the laptop. I would say that the biggest disadvantage of the laptop is the size of the footprint of the rig. I have a standard 14" laptop and my case setup is a little larger than I would like when the crowd is packed but have done it.
I like DAT but HDD based recording on the laptop has alot of advantages. The higher bit depths for one, but the biggest is not having to transfer from DAT to PC or whatever for burning to CD-R (moot point if you won't be trading etc). Also you have a piece of EQ that is multi-functional. You can record shows, surf the net etc if you want to so you ae not dropping tons of $$$ just on taping gear.
If you are getting a laptop just for taping I would do as advised above and wait for the HDD recorders to come out and go that route.
If you were getting a laptop anyway and were also gonna tape with it most new machines have enough CPU power to handle the job. I personally run a P4 1.8, 1024MB RAM, 60 GB HDD.