Regular readers see everyones sigs and tend to ignore them after awhile (not like I'm clicking on any ARS banners these days
).
Sigs are mostly going to catch casual or new readers (with the exception of a prolific poster changing their sig) so I would think having as many people posting your ad as possible would bring more signups. BUT, with that comes the risk of backlash from the regulars of the board. If that's not important to you, you'll probably do well, but if it is you should consider going with fewer high paid.
