matteocontrini That will be different from forum to forum.
This particular forum (of mine) has experienced some hiccups over the past 5 years and so only 11% of my traffic is from returning users.
The other thing to consider is that "new users" might be more likely to click ads whereas regular members might be more "blind" to the banners. So there's still a very good chance that a forum could make more money from showing ads to guests than to regular members.
Of course, none of this theory matters. The only way to know is to test.