I never owned a f1.8 but tried. I have a f2 but guess i lost it. These are what I observed after using a friend f1.8.
N35f2 is not as sharp - f1.8 sharper
N35f2 uses camera motor - noisy (for the case D70) I have no idea about D90 but D300 and D700 is alright - f1.8 it is SWM.
N35f2 focus ring turn when focus - f1.8 dont.
N35f2 is f2 - f1.8 is 1.8
N35f2 FX & DX - f1.8 DX
N35f2 cost more - f1.8 slightly cheaper
N35f2 come with depth of field scale - f1.8 dont but today too many people do not know how to use it
N35f2 focus down closer - f1.8 slightly further
AF speed on f2 depends on your camera motor speed too, but it is on par or slightly faster than f1.8. I feel that f1.8 is slow for a prime but if you use TC, f1.8 will have a better speed performance over f2.
If you are not going to upgrade, stay with f1.8 it is better.