A few days late here.
The 3.7 is a fine motor no doubt, but the output of the 5.0 is nothing to sneeze at. I have seen a lot of positive reviews for the larger engine. My preference is always V8s.
That being said, I believe the V6 will be cheaper and more available.
Especially so if you strictly want a QX70 (versus the FX nomenclature). With the introduction of the QX70 in 2014, that was the last year the V8 was offered.