Florian,
In the case of a point-to-point link, it is always sufficient to specify the outgoing interface. In the case of a broadcast/multi-access interface (like Ethernet), Cisco’s best practice is to specify BOTH the interface AND the next hop IP address. While in most cases, you can get away with using only the next-hop IP, there are a few circumstances where this can result in unexpected behavior. Cisco has a pretty good article explaining how you can get into trouble without specifying both here:
The take away from the article is this:
ConclusionCisco highly recommends that you specify the outbound interface and the next hop IP address when you configure static routes. When the outbound interface is a point-to-point type of link (for example, a serial link), the specification of the next hop address is not needed.