It depends on the dish.
On an 80cm, it shouldn't make much difference. On other sizes, there is more freedom to tweak the LNB positions to get the optimum signal.
Having said that, I have seen a monoblock on a 60cm dish working acceptable well too.
Also, if you want to feed in another sat, eg 28E from a Sky dish for example, then the best solution is to go for a 4x1 diseqc and two separate LNBs.
EDIT, just to clarify.
A monoblock contains two integrated LNBs and a 2x1 diseqc switch INSIDE it. An EXTERNAL diseqc switch is not needed and not of any use.
As SMM says a Twin monoblock is for two receivers, still no external diseqc switch needed.
An external Diseqc switch is needed, when you have two or more, separate LNBs.