Having used both for a month:
N80 Positives:
1. WiFig. This means upto 54Mbps if you are near a hotspot. No camera is going to take a picture better than that. Browsing the web, listening to online radio, checking email, using Gizmo and MSN messenger; oh and making calls too.
2. Screen resolution is amazing. Best I have seen of phones (and I volunteer at a mobile shop).
3. Slider makes it easy to make and receive calls (slide open to receive call, slide close to end call).
N80 Negatives:
1. Battery life. I know its been hammered; but if you are careful with what you do, you can make it last for a while. To improve it; adjust the light sensitivity slider and screen brightness (at full brightness, the screen really is amazing). Also turn off the wifi scanning (this takes a lot of power).
2. Keypad. For my fat fingers, the keys are bit too close and not separated enough.
N73 Positives:
1. So damn light! Its almost featherweight compared to the weight of the N80.
2. Camera. No more needs to be said here.
3. Compared to the N80, battery seems to last forever.
N73 Negatives:
1. Keys and joystick. The two that I used, the keys were too smooth and flush and it was difficult to avoid double pressing. Joystick -- frankly, I was afraid to use it much for fear that it would snap off
2. The camera slider mechanism is a bit loose. After a few photo sessions; it starts wobbling and looked like it would snap off. Had the same problem on two devices that I had.
If you don't need the WiFi, go with the N73. You will not be disappointed with either phone; they are both great.