Q: "The revenue data I see in my Google Analytics reports for paid search and display advertising is different from the revenue I see in AdWords. Which data is more accurate?"
A: They are both accurate. The difference is in how revenue (or conversions) is attributed. Understanding this difference can help you to determine which number is a better indicator for your efforts.
So which is better for you? It's really a question of what you are trying to measure. Are you more interested in seeing how you are doing purely from an advertising perspective? Then focusing on the revenue data directly from AdWords may be enough. You will get a sense of how well the ads are doing on their own to drive revenue. This is because the revenue (or conversions) will only be counted IF someone has clicked on an ad and at some point during the conversion window comes to your site and makes a purchase.
A good scenario in which to focus on data directly from the advertising engine is if you are only running ads to a siloed off landing page—a landing page that can't be found via organic search. In this case, you really are depending on the advertising to do the heavy work and aren't expecting to get too many conversions via other means such as people sharing the page.
Keep in mind that even in this scenario if the person comes to your site one day past the conversion window, their purchase or conversion will not be reflected in the AdWords revenue or conversion numbers.
The conversion window in Google AdWords is by default 30 days, meaning that if someone clicks on an ad, comes to your site, doesn't make a purchase but comes back before day 30 to make a purchase, a conversion will be registered. (The conversion window can be altered to account for longer purchase cycles.)
Attribution in Google Analytics is more complex, but will give you a fuller picture of how all channels are working together to drive revenue.
In all reports, except the AdWords and Multi-Channel Funnel reports, attribution is given to the last non-direct channel.
It works like this: Someone clicked on your search ad, came to the site, made a purchase. Paid search gets the credit. But let's say someone clicked on your search ad, came to the site, didn't purchase, then did another search and clicked on your organic listing and then made a purchase—the organic search gets the credit in Google Analytics, even though the paid search ad gets the credit in AdWords.
If you were only looking at the conversions in Adwords, you might miss the fact that organic search is the closer while the ad is only assisting in the transaction. That may or may not be an important nuance for measuring your effort.
