If I display Google ads for all users, guests and members, I get a very high impression count, but a low CTR.

If I display Google ads for only guests, I get relatively low impression count, but a higher than average CTR.

Which is better?

All that mattes is the bottom line, the resulting revenue. If you are showing your ads to both members and visitors, and have a high volume of page views, you should have a noticeable amount of CPM revenue to go along with your CPC.

I don't think CTR has any ultimate bearing on your revenue though.

That's what I was wondering.

I'm turning off my ads for members. This will drastically lower my page impressions, but not affect my clicks. I want to see what happens to my earnings when my CTR goes up 20 times what it has been (clicks remaining basically the same).

I agree with Mike

I'm going to run a half banner image ad for the members just to see what happens.