2CO is more reliable compared to paysystems. I myself considered to use paysystems due to the cheaper rates but they have very poor merchant support and they are more client oriented. As in when your clients decides for a refund the clients just have to request from paysystems and they will get it. You will not be informed unless you log in a check it personally. 2CO however when a client request for a refund it, you will be informed about it before any refund is made. So its a two way oriented. Therefore I personally feel 2checkout is much better. I have been using 2checkout to pay since 2002 and it has not failed me once and I am getting a new merchant account with them.
I just thought I should share it