3rz hands down. 4cyl aren't balanced but neither are v6 engines. The balance engine is an inline 6 if I am remembering everything right.

Both are good engines but I prefer the 4 cyl

Beamed from my dumb phone using Yotatech

 

I would go with the 3.4 V6. I haven't driven either of the gasoline engines in an early truck, but I prefer the smoothness the V6 will offer.

My 6th gen single cab 4x4 truck is a 2.5 D-4D, so not directly comparable. It sucks though, hence looking to replace it with Lexus power.