Yup, I drove the 350 and the G35 Coupe before I bought the G. Here's my take. The 350 is a bit faster. It's lighter and you really feel it when driving. Switch off the traction control and you really feel how light it is. The 350 only comes in a 2 seat version. The 350 Roadster holds it's value better than any other convertible on the road (within reason.... I'm not comparing to Porshes, Astins, etc...).
Here is why I bought the G Coupe... It's almost as fast, rides better, handles almost as good, better looking in a hardtop (JMHO), MUCH nicer inside, it is a 4 seater with a big back seat (go look at one you will be very surprised) and Infiniti's service is second to none. I drop the car off for an oil change and they have a G35 Sedan waiting as my loaner. We have 2 kids and when the Burb is in the shop the 4 of us still need a car. Drove it to Nashville and no complaints from the little Otters in the back.
If I didn't have kids or a need to drive them around as often as I do I would have bought the 350 Roadster.
I have 34,000 on my Coupe and love it. Thinking of trading it in for another coupe. They have a 6 speed with Navigation on the lot right now

. Mine is a 6 speed but no Nav.