You are not the first to suggest, but for now they always take the fastest one for performance reasons. Different travel behaviour multiplies pathfinding, which already now is the "pulled handbrake" in the game when your network grows. Hopefully the developer overcomes this like so many seemingly insolvable obstacles before.
Until then you can use restrictions "only board" and "only exit" or minimum travel times on the fast service to control travel behaviour at least to a certain extent.

Well, you could adjust your express line, where you don't allow pax to enter/exit at certain stops, or you can define a minimum ride time. This will force some of the pax to the regional service.