I only recommend using credit based safelists.
There is one simple reason for this advice which is that when I receive email from these credit based safe lists, I have to click on the link and visit the web page being advertised to earn credits, so that I can then send my own email adverts.
With any other type of safe list I just ignore the messages as they go to a list email that I just empty every day so they are pointless as a marketing tool, as everyone else will be doing exactly the same!
This is the way I tend to judge most of the advertising resources I use, as if I actually see and respond positively then a lot of other people will too.
If you want to speed things up a bit and have the funds then I highly recommend Referral frenzy and Traffic Zipper as they will really speed up your Safelist mailings.