I understand this is for just one day? You could split the time equally between the villages. I personally prefer Positano but if you have never been to either, I'd just do half and half. Go to Amalfi first then go to Positano. If you travel on the SITA bus from Amalfi to Positano, get off at the first top (the La Sponda stop) in Positano as it will be the closest to the pedestrian only portion of the village which leads down to the beach/seaside.
If you end up stopping Positano first coming from Sorrento - the La Sponda stop will be the second stop in Positano. If you get off at the Bar Internazionale stop, you will be very high up. You could, of course, then take the little internal bus that runs along the one, one-way road in Positana, Viale Pasitea. It drops you off at the top of the pedestrian stop, Piazza dei Mulini.
Do some google mapping and you will see these places on a map - it's all small so you won't go wrong whichever stop you get off at; it's just that the La Sponda stop is closest to the heart of the village.
There is a beautiful cathedral in Amalfi and the village road runs through the village with shops and restaurants along side. The buses are right near the harbor.
There's a nifty paper mill museo to visit: amalficoast.com/primopiano/paese/amalfi-2/am… (This site also has some other info about Amalfi)
and some more info about Amalfi:
And, a blog by someone who lives in Amalfi:
Some info about Positano:
A daily photo blog by someone who lives in Positano:
Hopefully, some of these sites will help you decide how to spend your very brief few hours there.
Edited: 11:45 am, May 15, 2013