A big factor I think is which type of music you listen to as each will accent them in different ways. The S4's come with the three sizes of single flange tips, the A151's have the three sizes of single flange, a set of double flange and a set of triples. Seems the S4's are easier worn cord down, and the A151's are better over the ear. They're both 3mm nozzle size, which is good if you have narrow canals like I do. I really like the A151's cord though, pretty much no microphonics from it, even when walking or running.
Take a look at
this thread if you haven't, and search for codes:
(2C3) - for the S4
(2C26) - for the A151
Main thing I've noticed is the A151's go more for accuracy, while the S4's have a more bass heavy sound. It's just a matter of which you prefer.