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.
Daniel R Hicks's comment is correct: You should be able to go 3mm smaller with no problems. It's likely that you could go significantly smaller than that. Eventually, you hit the possibility that the rims are too narrow for the tire but unless you're running rims that were designed for a 29er, you're not likely to have that problem.
It's also not likely that you'll notice much of a difference in ride quality based solely on the size change. Dropping from 35 to 32mm is really such a small difference that any change that you notice will more likely be attributable to changing the brand, model, and tread pattern.
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.