I'd rather have the IS350 than the G37 Coupe. While the G37 packs a stronger punch (330 hp vs. 306 hp), it just seems too similar to the G35 Coupe exterior-wise. In addition, it's gotten so-so reviews and has been having trouble keeping up with the BMW 335i in various tests, except in one publication where Infiniti brought along a 335i to test, but it was on a track of their choice, and readers suspected that it was a track that favored the G37's dynamics. The IS350 isn't a BMW-killer either, but it seems a lot more of a grown-up car than the G37.
But realistically speaking, I wouldn't buy either of those. BMW 335i all the way, if I were looking for something along the lines of sporty and semi-luxurious. That engine is hard to beat.