Can only really speak about the 3.0 (it's what I have in my Trol :thumbs)...
3.0 has more power and better mpg BUT when Nissan first developed it for the Trol they made a hash of the engine design and there were major and terminal issues with them, engine had a bit of a re-design and consensus is that it was all solved by the time they popped them in in the Terranos (mid 2003 ish). Long as you don't muck about with the engine (like fitting large exhausts etc) you're fine but as a result some folk are wary of them so I imagine that's why there's a few more of them around :nenau
Also, I don't think the 3.0 will run on veg quite as easily as the 2.7 and the 3.0 has a timing chain as opposed to the gears on the 2.7 which might put folk off the 3.0 too.