dbuckley: The question of 3 pole or 4 pole has been bothering me all day. Ill go with a 3 pole
Just to add some clarity to this; the only common scenario where a four pole breaker is
required is where you have one generator backing up multiple services: Each service has its own N/G bond, and if you add a single genset to two or more services then you need to interconnect all the grounds (nothing wrong with that) and all the neutrals. And it is this interconnecting of neutrals that causes problems.
Essentially, due to the presence of multiple N/G bonds, you have neutral current flowing deliberately through the ground wiring. So you need a four pole switch after the service entrance and after the N/G bond, and then switch all four poles to either utility power or to generator power. In this situation the generator has a N/G bond.
In the simpler situation of one generator backing up one supply, the single N/G bond is all that's needed, and there are no consequent nasty currents on the ground wiring.