Thanks for the reply. I agree with shanonm on this one in that while it looks like the "main" difference between the two emails was having the button there were actually a ton of changes made. Because the one thing that can easily be measured is how many times the button is clicked, that is the first place to start.

That said, it also looked like that was the only way to see if the changes worked. So the button itself could have made your percentage change down, but the rest of the ad make it change dramatically up, so overall the effect is up. But, if the lesson you learned was button = less CTR and didn't test the rest of the changes, it might hurt you in the long run.