3 wheels all the way if you want to carry a serious load. Also negotiating traffic on a 2 wheel cargo bike is plain hazardous.
On the other hand:
3 wheels are much, much slower (3 tyres = 50% increase in rolling resistance, much bigger aero profile, increased weight). Also, 3 wheels are inherently much less stable, so cornering is a pain. A unladen 2 wheeler is just a much longer bike, a bit slower, but still fun, whereas a 3 wheeler is always just really slow. Unless you are regularly (say, at least once a fortnight) carrying loads of 50+ kgs, I would say that one should steer away from trikes.
On the other hand:
3 wheels are much, much slower (3 tyres = 50% increase in rolling resistance, much bigger aero profile, increased weight). Also, 3 wheels are inherently much less stable, so cornering is a pain. A unladen 2 wheeler is just a much longer bike, a bit slower, but still fun, whereas a 3 wheeler is always just really slow. Unless you are regularly (say, at least once a fortnight) carrying loads of 50+ kgs, I would say that one should steer away from trikes.