To buy or to Trade?

web20

which is the best solution for links? To buy them with only 1 way links or to trade them? And why?
Are blog links more powerful than normal sites?

Example:
I had a link exchange program and had about 500 links /month with any kind of website or PR...i had the link pages with nofollow and .htaccess restricted for robots
I've got PR3 for all my site pages--about 15...even for the portfolio ones

I've stopped that and bought some PR3-4 links
After 1 month google sent me to page 4 instead of page 2 for web 2.0 design keyword, and even took all my pr leaving me a PR2 for the homepage..

Strange right?

Therefore

I think that you know your goal. I think that buying links from other web sites worth it. At the same time you need to continue your daily manual SEO