I vote 3.5L ... I had the 3.0L in my '97 I30 and I've got the 3.5L in my '02 I35. Gas mileage is similar.... here is a list of the cars I've owned and the gas mileage they got, all in the same location with similar driving (60% hwy, 40% city). This is the average MPG after tracking many fillups and how many miles I traveled according to the trip.
1989 Pontiac Grand Am - 25 mpg over 12 months
1998 Saturn SC2 - 26.5 mpg over 13 months
2002 Honda Civic - 28 mpg over 18 months (was rated for 30 city, 38 hwy)
1985 Toyota Celica GT-S - 23 mpg over 4 months
1997 Infiniti I30 - 24 mpg over 12 months
1998 Infiniti QX4 - 15.5 mpg over 9 months
2002 Infiniti I35 - 22 mpg over 3 months
The difference between 2 miles per gallon means less than $100 in savings over a 12 month period. I'm happy with my 3.5L
Having said that, I'd be happy with either a 3.0L or a 3.5L VQ... I love them. However, if you don't mind a 2 mpg deficit, pick up a 3.5.