Which is better?

Well, more people equals more content, and that is good for SEO, assuming the content is worth reading!

Well if you have the budget to hire good writers then it's better...

SEM and back links can't compete each other since they have their own specific purpose.

1. Not every keywords convert into money. This is why SEM give you more option to see what keywords really worth in the long run.

2. You can't always control all the keywords to rank in organic result. Please notice that I say "all" which mean that you can control some keywords to get on the top of organic result, but before participate in doing back link to get this result you need to know that...

- Brand new website can't get on top for short-phrase keywords (Sandbox).

- Ranking on top of organic result for wrong keywords that don't bring in any conversion and money is a waste of time!!! (Don't let the search queries report from Google Keywords Tool get you wrong).

Summary - traffic is not equal money....filtering traffic and only focus on the good traffic is the way to go.

If you get only 100 visitors per day but you can covert 10% of them, then you are the rock!!! (your traffic will convert more if you start a mailing list)

Blogs that has many writers are always in advantage...