Typically we use the continuous duty relays, which are actually contactors, to throw the battery onto the power bus, and we use a slightly lighter (1 oz less) and slightly cheaper ($8 less) intermittent duty relay as the starter contactor.  My particular installation would be aided by using the metal-can continuous-duty relay as a starter contactor -- it's all about existing mounting holes and clearance to adjacent equipment.  The answer would be simple if I knew the rating of the two devices (these are the parts out of Aircraft Spruce or Aerocraft -- the usual suspects), so theoretical answers are not what I'm looking for.