I own a 2003 330i SP 5-speed, and have driven a G35 sedan extensively. The G35 handles and goes very well -- chassis and engine dynamics are true sports sedan caliber. It has lots of room inside and is cheaper than a 330i. Despite many people's complaints, I personally I thought the interior was OK. The perforated leather seats are superficially nicer than the 3-series. The top-mounted seat controls didn't bother me. However the driver's left armrest was very hard and uncomfortable -- like vinyl over metal.
However overall it's not as refined as the BMW. On paper the G35 automatic sedan is roughly as fast as a 330i 5-speed. Actually driving them feels very different. The G35 has an exaggerated throttle tip-in, making it initially feel very fast. Then progressing to full throttle, not much else. Then at about 4500 rpm, a big kick. At higher rpm, engine is much louder than the 330i, and a little rough. At low rpm there's a V8-like exhaust rumble that's either cool or irritating depending on your viewpoint.
By contrast the 330i power delivery is very smooth and linear. It's torquey down low and smooth as silk at the redline. It takes more throttle angle, plus engine is quieter so may viscerally feel slower than the G35, but they're about the same. I wouldn't choose cars based on acceleration this close.
Handling-wise G35 has good turn-in and grip. Nice, fast steering response, yet not darty. The brakes can feel grabby, but you get used to that. However chassis isn't as communicative, and above 8/10ths in corners you're not as secure. But the highway ride is better and it tramlines less than a 330i SP. Infiniti did a great job on chassis and engine dynamics. Overall it's a very nice driving car.
But here are some G35 missing refinements: no telescoping steering column, no fore/aft adjustable headrests, no lighted steering wheel buttons, no self-leveling xenons, no adjustable thigh support, seat heaters slower and less coverage, nighttime instrument illumination either too bright or too dim, instrument lights can make distracting reflection on windshield. Individually these seem small, but collectively they make the G35 cockpit less nice than the 330i.
Another example of better 330i refinement is in traction control. If the G35 VSC detects wheel spin, it just kills engine power. Feels like someone turned off the key, it's so invasive. By contrast BMW's DSC uses automatic differential braking (ADB) which individually brakes the slipping back wheel. It's much less intrusive.
But despite any G35 flaws I highly recommend anybody in the market drive it. It's a good car and you get lots of bang for the buck, and plenty of interior room, esp in the back, plus good trunk space (sedan only). I think it's the best 330i sports sedan competitor, excepting maybe the new RX-8, a much smaller car. But if you don't need much back seat leg room, drive the RX-8, also.