I don't know about the sound level in operation, but according to Airexcel (Suburban) the amp draw is less on an NT.
NT-30SP draws 4.5 amps
SF-30 draws 7 amps
power consumption is closer in the high-BTU furnaces, but still slightly lower for the NT:
NT-40 draws 8.5 amps
SF-42 draws 9.0 amps
It looks like the SF units are slightly cheaper, but not a big difference. $10-$15 difference in the sites I checked.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill