The Nokia 6310 is essentially an update of the 6210.
Its got the same form factor and weight. The keys are a bit larger than the 6210 and the six control keys are spread out from each other. Unfortunatly this means that the up/down button has now become two buttons with a small bar in the middle, it takes a bit of getting used to from the 6210, expect your thumb to hurt for the first few days you have this phone. The screen has a slightly higher resolution and now can display four lines of your address book in name view instead of three. Alot of the software bugs in the 6210 have been ironed out so it feels slightly faster, especially when you open up your SMS inbox when its quite full.
The 6310 also has a much improved battery life. Nokia claims that it has a standby of 18 days. I only tend to recharge mine once a week but YMMV. It has bluetooth built in, an IR port and WAP 1.2. It has a built in voice recorder also, as well as some voice commands to do common tasks such as changing profiles.
All in all its a great upgrade of an already great phone. I'm very happy that I upgraded my year old 6210 for this newer model.