Tom, I use the SE 5 kit but I don't have experience with the others, so I can't help with a direct comparison. You can keep the SE 5 going for a long time by judicious use of the additives with a bit of replenishment, but what I like is that you can play with them to get quite different effects from the same paper, from smooth and creamy to very grainy and lithy, just by changing the amounts and the dilution. The extra ingredients give you more control, although also more to experiment with and a consequent chance of getting it wrong. Presumably you could do the same with home brewed additives in the other developers - as far as I understand C and D are just sulphide and bromide additives to give independent control of the levels of each in the developer.
There is also a Ammonium Carbonate solution (Omega additive) which is supposed to increase colour intensity.