I need a 21" and am undecided about model. So far it looks like a lawnboy, self prop, mulch/bag attachment. Question is 2 or 4 stroke. I know the major difference being oil/gas mix but what are other pros and cons. I only have a minimal need for the trim but it is a need. Z won't fit and too much for whacker. Should i try to get away with a cheaper brand? Is the 5 banger worth it?