IMHO
If you have a high traffic site and want to manage the ad sales and placement yourself to maximize revenue (you could collect full billing instead of just a percentage commission)then Ad Server looks tremendous!
If you want Google to do the work and place ads for you, then the Adsense block is a "set and forget" method that pays a commission on the clicks.
A third option is somewhat between those two add-ons. With Google adsense you can choose to display the advertiser of your choice and just place their ad info in an HTML block for free (more work for the commission unless the ads you choose are a great match for your site audience)

That's good to know. I would be looking to getting advertising myself and then fill up the rest of the site with Google Adsense.
Is that possible? To work Google Adsense and Ad Server on the same site?

I've never used both on the same site so the following is a guess: I believe they are compatible as they are two totally different blocks... perhaps a pre-sale question from the marketplace to the core team for Ad-Server would be best as that may save you some money if it will do both. Good luck on your project and post your success please!