Which is best 2 or 4 port and why?

Or does it depend upon the heating load types (e.g. underfloor / simple zone controlled rads)

So under what circumstances would a 2 port connection be best and when woould 4 port connections be best.

Adding to that in each scenario which is the best way to send the call for heat to the HP?

Finally why use a buffer tanks instead of a loss loss header?