I guess that all depends on the name itself and the market to which your domain is aimed at.
Personally, if I had a product aimed at both maybe US and UK and the .com was taken I would choose the .co.uk over the .net and aim my product at the UK. But, like I said, it all depends on your domain/product.
The question about value of .net, like the above it all depends on the name. You could 'try' and catch a dropped LLL.net and sell for low xxxx. Or you could register an ok .net and sell for low xxx. Everything depends on the name itself and the market its aimed at.
.com = commercial
.net = network
.org = organisation (organization US) ha
Dan