M3 is 50% faster. 48 to 32 mph. "Need troops town A55 quick!" ... 
In the case of the M3 vs SdKfz 251, if the troops are an after thought and are being reactive, then the M3 is the better call because speed is of the essence. But, if the troops carrier is a part of an assault force of tanks, etc, then the 251 will certainly get the nod from me because it does seem to shed enemy fire better. There is no sense in having the faster M3 if it has to stay with or behind the tanks. Say... that sounds similar to what the Germans did at the start of WWII.
