So out of the four sites I review, what is the best for me to use based on their payment method options?

Well, as you might expect, each one allows you to pay slightly differently.

MP3Million  
This site takes MasterCard, Visa, Visa Electron (handy for the students among us!), Maestro, uKash, PaySafe and DaoPay. The minimum top up is €19.95.

MP3va 
MP3va accept MasterCard and Visa only. They did offer PayPal, but this has since been removed. I have contacted them about this and it is due to be added soon. Watch this space! The minimum top up on both sits is $14.95.