With no baggage? I'd go for the kiosk, if I didn't already use online checkin. Remember that nwa.com promotions like the 100,000 Mile Dash give you extra points for OLCI to help you qualify for bonus miles. In the new promotion, I think the kiosk also gives you some points, though not as much as online.

If I didn't already have an upgrade clear, I might use the counter to ask where I was on the list.

Actually even with baggage, I'd use the kiosk if I was getting extra points. Sometimes the agents at the First/WBC/Elite counter aren't working all that fast - or are taking extra time to give full service to somebody, so just using the kiosk can be quicker.

At BOS, there's a kiosk specifically for FC/Elite. At DEN, there's just a line. I find that BOS more quickly works the FC/Elite line for people using the kiosk, while at DEN the FC/Elite line is sometimes slow.

If your question is specific to NW, I think the mods should probably move this to the WorldPerks forum.