I have used Article Post Robot and am using Article Marketing Robot now. Comparing the two, i would say of course that Article Marketing Robot is a better one, in fact, one of the better ones in the market really.
Article Post Robot was created about four years ago if i am not wrong and it did serve its purpose then. The last update was 2009 and when i tried to use it some time in 2010, it was not responding. Support took a while too before i got the system up again. I suspect that the key issue with APR is that it has not been maintained for a while now, not to mention that there are only less than 500 article directories in the system.
That is my take on the comparison. I have used the other two mentioned in the rankings.