Which Conversion Model is Right For Me?

If someone finds your website through a PPC ad and immediately buys something, it's obvious that the ad and keyword that generated the sale should get credit. But what if a customer comes through multiple ads? What if someone came to this site first from the keyword "PPC advice blog", then comes back on the keyword "PPC management"? Which keyword should take credit for this sale? Google has now made it possible to change your attribution model, but which one is for you?

1) Last click attribution: All credit goes to the last keyword before the sale. This is the default setting and my preferred setting because PPC is best for "closing the deal" for customers that have figured out what they want but are searching to figure out what company to buy it from.

2) First click attribution: All credit goes to the first way a customer finds your website.

3) Linear: Each keyword gets equal credit for the sale. This way every interaction gets some credit for the sale.

4) Time decay: Similar to linear, except the most recent keyword gets the most credit. This combines some of the benefits of linear and last click.

5) Position-based: 40% of the credit goes to both the first keyword and the last keyword, with the other 20% split between sources in between.

6) Data-driven: Credit is assigned based on an analysis of how important that keyword is to the conversion. Google will analyze all paths and determine which keywords are most important to the sale.

I have never used the data-driven model because it is a relatively new feature, but I am definitely interested in testing it out. For most accounts I still prefer the last click attribution; when I'm spending money on a PPC campaign I am looking for people ready to buy a product, not one who (hopefully) will come back to the site later on. What do you think?

For more information, here is the Adwords help page.