I only have experience with the 2.7. It's a very nice motor. Very worthy of replacing the 22RE. Lots of power and torque for it's size, although Toyota gave a bit of that back with the higher gearing that it put in the Tacos.

If simplicity of maintainence and fuel economy are concerns, then I would go for a 2.7 liter, but if performance is what you want, I've never heard anything bad about the 3.4 except for some headgasket issues in the very early years.

I don't know what the real world figures are, but EPA estimates put the two engines within 1 to 2 mpg of each other, which to me would give a pretty clear advantage to the 3.4 V-6 since it makes so much more power.