tyre size for a 28" bike, at the moment: 35C, is 32C too narrow? since my tyres are a little old and won't make it much longer, I will buy new ones. At the moment I have Schwalbe Marathon 700-35C  and I was thinking to replace them with a Continental Contact II 700 x 32C. Since there are no Contact II 700 x 35 C I thought about going slightly thinner instead of 700 x 37. Any Ideas on this?

Having less friction on the tyres could be great since my commute grew considerably longer this year.
I doubt the rolling resistance or air resistance will be much changed by using a 3 mm narrower tyre.  In terms of compatibility there's a chart here where you can check your rim size against safe tyre combinations.  Although as it says below, many people go outside this range without problems and I have done so myself.