Hi
You ask a good question. This question is similar to when startups ask me which would be the best business model or pricing mechanism.
My answer, which might simple simple (but read on) is: both.
People like to have the option to choose. It lets them feel more in control, especially when having to pay for services. I have personally used seo services in the past, and never agreed to pay for the per month option as too many companies were abusing this option and it comes with uncertainty for the client.
So, what you should do is create two payment options: one based on results (keyword ranking) and one based on the per month method.
A few important things to note when doing the above:
1. Do not offer the results based option if you don't think that you can reach results due to limitations/conditions on your client's websites.
2. Make sure the results based offer reflects all your costs, the risks and the desired profit that you want.
3. Specifically for SEO services, make it clear (upfront) to your client that they will need to make certain changes to their website for your service to work.
There are a few other issues that you need to cover. I will be happy to discuss these by phone.
p.s.: if you added the link in your question for SEO purposes, then: on the one hand - good job. On the other hand, for people who understand SEO, they might think that you're using black hat methods and then decide to refrain from using your services.
Good luck