The biggest competition in your niche didn't register the .net version of his site. What's your play? I guess this might be an ethical question but I'm sketchy on the big picture here. What are your options?
You could buy it and just use google adsense domains for it, if you had it redirecting to your site he'd know it was you
Is it a name or a keyword. If it's a generic keyword register it. If it is an actual name of a company, I would register it. IE: OilChange.com - I would OilChange.net IE: MrLube.com - I would not register MrLube.net I do not really have ethical issues with registering it, as a customer I wouldn't think it is that classy, is the only reason I wouldn't do it.