As can be seen from the download page, 5.0.2 is currently a GA release while 5.1 is FCS. As with any newer release, 5.1 has additional features that will not exist in the 5.0.x code line. If a customer has no preference or particular requirements then they should start with the NGS recommended release. That guidance is provided for those who might need it and is not intended to be used as a mandate.
The correct choice is to always to use whichever version supports the customer environment/needs the best.