It depend on what road surfaces you intend to travel. If gravel/dirt is in there somewhere, a treaded tyre such as the Marathon may well be your better choice.

However, we have been using Schwalbe Durano slick tyres for the past few years on our tandem and touring bikes (although the tourers still get mixed with Marathons). The Duranos have been in 700 x 28C size, although we also have used some 25C, too.