Almost always a laptop. Often with a separate USB keyboard. I can set it up as a desktop, and then unplug it and haul it to writer's meetings, critique groups, or to the park.
For the last 6 years it's been
this Gateway laptop, for both work and writing. It's been a great workhorse. Great video, full size keyboard.
The next one is going to be smaller, either an
ASUS or the
HP Mini. I've test driven both, and been equally impressed with their performance. Both accept the separate USB full size keyboard. (The small screen is fine by me, but the keyboards are too small for speed writing.)
The change is a desire for 1) longer battery life, and 2) more portability. (The 675 isn't portable, it's luggable.)
A note: Both are available with Linux or Windows operating systems. If you aren't used to tinkering with the hows and whys of computer operating systems, and you don't have a personal geek to count on, I'd recommend sticking with the Windows OS.