Im having fun with the Sigma 30mm F1.4. I have not tried the others but the 30mm would give u an angle like a 50mm on a FF. 50mm on D90 is rather tight. The 30mm is excellent during low light conditions and it focus really fast. I guess it depends on what kind of shots you wanna take which will give you your choice between 30 and 50.
30 - Slightly wider, good for a group of maybe 4 pax with half body shots? 50 would be tighter. If im not wrong, 50mm on a D90 will come up to about 70+mm. You can take your kit lens and adjust to 30 and 50 to see the differences. Or you can rent the lenses before buying. Both the sigma 30mm and 50mm are really great lenses.
Do correct me if im wrong. =)