997 has much better interior. The exterior (especially front) looks better too. 997 has classic 911 looks. 996 with the "fried egg" headlights never looked quite right. I would get the 997 C4. Rule of thumb: always get the latest Porsche model you can afford.
I second what Silver Bullet said. The 997C4 weights the same, has 5 more HP and the Tip might have been a bit improved. It should be a little faster, but nothing that you should notice. It looks much better both outside and inside, though.
I do not know about the tiptronic, but the regular 997 with 6 speed is just a bit faster than the 996C4S with 6 speed. They are so close that depending on the driver either one could be slightly quicker. But generally speaking as I have driven both the 997 is slightly quicker and the 997S even better