What is the maximum voltage for MPPT/PWM to charge a X volt battery?
At the risk of stating the obvious, the MPPT is always going to do a better job.  It is "gaming the panel", making experimental adjustments upward or downward in current draw to see if that improves or worsens voltage enough to produce a net increase in V*A.  

In theory, both of them, MPPT or PWM, should buck the input voltage down to an appropriate battery charge voltage given the stage of battery charging, while preserving W as much as possible through the bucking (i.e. if they buck down from 48V@4A to 24V, that should be about 8A).  

However, I have been told by solar tech supports that PWM converters do not buck, and that it is impossible for output current to exceed input current: So if the panel is outputting 48V @ 4A, then the battery charge will be 24V @ 4A.  And if I want the current-multiplication of bucking, I need to go with an MPPT controller.  I'm not 100% sure I believe that, but I haven't been able to put meters on my setup.