If you're looking for fuel economy, the 2014 Lexus RX 450h is the better choice here. It's one of the greenest family-sized vehicles, and with its well-coordinated powertrain and a pricetag that isn't all that much higher than the RX 350. EPA ratings place the RX 450h at up to 32 mpg city, 28 highway, but the last time we drove an RX 450h, over mixed driving conditions, we didn't see anything close to that EPA rating—about 23 mpg overall, with all-wheel-drive model (rated 30/28).
Standard RX 350 models are a little more consistent with other mid-size crossovers in the segment. Keep in mind that it does only have two rows of seating, for a total of five, at a time when many vehicles around its size are offering a third row. But the key is that you're likely to use the second row on a regular basis; this is a vehicle that can carry four adults around in comfort. And that makes the RX 350's fuel economy--in the upper teens, as we've seen it--not all bad.
All RX 350 models now include an 'eco driving' light that simply highlights when you're driving mindfully.