briggidere - 6:51 am on Dec 17, 2010 (gmt 0)
If you have multiple ads in one adgroup the adwords system will start to automatically start to show the better performing ads more often (based on CTR). This is the default setting and they call it optimise.
You can set it to rotate more evenly if you want to split test the ads with more even impression data.
The higher CTR ad may not necessarily be the best performing ad so look at the conversion rate/cost or better yet ROI data for each ad for more accurate data to base you decision on which one is best.