Depends on your ability to wrench and what you want the truck for. A lot of guys around here wrench and weld for a living or have a lot of experience at it and, if you are one of them, a beater can be a fun project, as long as the rust isn't terminal. I'm not in that category, although I aspire to be, so, for me, it was better to buy a pretty solid cruiser right off the start.
If you want a DD, first off think if you really want to deal with an FJ40 DD (I do, and I love it, but the commute is a labor of love) you'll probably want a nicer truck. If you want it as a trail rig, a beater with a solid frame might be the right option, again, depending on your ability to wrench.
Given how much gas has driven prices for cruisers down in the last year, you should be able to find something for a decent price. Which is good, 'cause you'll need as much money for gas as you can scrounge.
Cheers,
prat