Positano special dinner choices
I have never eaten up that far in Montepertuso, but the recommendation I get over and over and over again is for La Tagliata.
In Positano proper, I love Bar Bruno on Cristoforo Colombo, 157, If the weather is nice you can sit outside (Note - it is along the road) and see all of Positano twinkling. The pastas are homemade and fresh with local ingredients (anchovies, mussels, local fish) and the desserts are to die for. Their twist on the traditional ricotta e pere is a standout.
La Sponda at Le Sirenuse is an incredibly romantic, candle lit, Michelin starred place.
Any suggestions for what? Are you looking for restaurant suggestions other than the 3 you listed above?
I will second the Bar Bruno suggestion, but only if it is nice enough to sit outside.
If it is nice weather I would also suggest having a cocktail at sunset on the terrace at Hotel Il San Pietro - the view is worth the short taxi ride.
I have not eaten at the 3 restaurants you listed above.