It's good to answer this question with the bases of comparison in mind.
1. As to RESULT - Paid ads is more effective because the site have already visitors that are always coming back to their site. It also have little competition compared to free ads therefore have better chance of getting affiliates.
2. AS to COST - Free ads are obviously better. But if you monetize the efforts that you do then paid ads may turn to be less cost.
3. As to EFFORTS - paid ads take lesser efforts that the free ads.
4. As to TIME to get Results - Paid ads can give you results in shorter time.
5. As to QUALITY of affiliates - It will be fairly the same, no one can guarantee the quality of sign-up in both.
Conclusion,
If you have money to do paid ads go ahead its the best and more effective. But if none, well, what can you do but to go to free ads.
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It's good to answer this question with the bases of comparison in mind.
1. As to RESULT - Paid ads is more effective because the site have already visitors that are always coming back to their site. It also have little competition compared to free ads therefore have better chance of getting affiliates.
2. AS to COST - Free ads are obviously better. But if you monetize the efforts that you do then paid ads may turn to be less cost.