What is better a .net OR a .com with a dash?
 

The title says it all.

For example which one would be better for surfers to find and for search engines.

rob-max.com OR robmax.net

Good question!

For search engines I don't think it matters to much, but for the second part of your question i'd love to see some others views.

why don't you buy them both and permanently redirect one to another?

doesn't matter at all.

they are equal for search engines

the only difference is when it comes to word or mouth or print advertising