I read something a while back that said .info and .biz sites were the most likely to be seen as spammy/scammy/virus-laden. As a consumer, I prefer to see .com sites, but .net sites don't bother me as much. Perhaps you can find something that is very close to the original .com site name you want and obtain that.
The biggest thing with .net is that you will lose traffic to people who remember the domain name part and forget that you are .net instead of .com. So they enter in .com, get a site that probably isn't relevant, and may or may not try to figure out what your actual domain name is.
I don't know how search engines weigh .com vs .net, so no comment there, but if you are thinking of spending money on SEO, take a step back and make sure you are hiring a web designer who understands basic SEO first, and spend your money there. A good designer will be handling basic SEO things naturally, but if they actually *understand* what they are doing, they can give you more bang for your buck than hiring someone different later to come in and try to layer stuff on top that should have been built in from the beginning.