The AdWords Ad.
The wording of the ad is of course a critical factor. Get it wrong, and either the CTR or the ROI will plummet.
As well as the words you use, you also need to consider how your ad compares with the others that will be displayed by Google.
The content of your ad will be seen right next to your competition, so you have to keep an eye on what they're saying, and you might want to consider how (or if) they're competing with each other.
You should also try to get as much accurate information in there as you can, while simultaneously making sure that the overall impression is pleasing and compelling. Space is a little tight though, but no-one ever said it would be easy.
If you have a free trial of your software, then this might be worth including too. But don't give the impression that the software is free. Again, you'll get great clicks but a poor return on investment.