Having two sites with different content, one at domainname.com and the other at domain-name.com, would seem somewhat strange to me.
Is a telecommunications/internet site targeting a mainly US audience.
If this is the case I would go with the domainname.net. It fits in well with telecom/internet sites.
Also, the biggest downfall of having the .net version is that most people will type in .com by habit. Since you also own the .com it won't be such a big factor.
I do agree with JohnCarpenter about avoiding the domain-name.com one.
I would register them all