I would go with the 4 pole xfer. A 120/208 generator system without a Xo bond could backfeed the line neutral looking for a return path to ground. This may be a safety issue if someone was working on the main service, with the generator running and open the neutral ahead of the bond to ground. The above post was VERY WISE about the problems with a GFI and a second ground. I was on a service call last year for that kind of problem. GFI trip as soon as the load exceeded the GFI trip settings. This took awhile to find due to size of the building.
I will try to explain...as you know the main service neutral ("grounded conductor" as the nec like's to call it) is bonded to ground at the main service. This is the only place the code lets you do this, unless of course you have a separately derived system such as a generator. Now if this generator is using this neutral without a X0 bond at the generator (and a 3p xfer ) any leaks to ground or faults to ground will return to the neutral at the main service bonding jumper ("backfeed" in electrician talk). This is were it could be a safety issue, if someone was working on this neutral and open it ( this is something some people only do once.. they don't get a 2nd chance). A 4p xfer switch with its own X0 bond at its neutral, would be isolated from the main service neutral and could not backfeed when the generator stated.
Hope this helps... Maybe someone smarter then me may explain it better.
I'm sure that there are many sharp people here, I'm here to learn as much from them as I can, and maybe share a little as I go, this is how I think it works from what I see so far.
I made the changes you requested to the picture you posted but I could not paste it. Maybe I did something wrong or maybe it was something I didn't do ?.
Lets try this. Lets say we have a small leakage to ground on the load in your picture ( a old motor maybe ), lets say its leaking to A phase. This leakage would follow the ground trying to return to its source . It would follow the ground path back to the main service to the bonding jumper, then on to the neutral, follow this back until it was cancel out by B phase at the generator. This system would work fine until someone open the neutral at the main service, ( maybe a main GFI test or a service upgrade ). When they turn main power off the generator would come on ( ATS in the 1st post ). There is no X0 bond at the ATS, it would be using the main service X0 bond for its ground path. Someone then opens this path and we hope they just get a shock... But that depends on how they open it and how much leakage we had.
On the 1st question I used the wrong words (terms) should have said the ground current is trying to complete the circuit thru the neutral via the bonding jumper at the main service.
2nd I agree with you 100% ( maybe more ). In this circuit the bond is at the main service so any faults to ground must travel back to the main service bonding jumper then follow that neutral back to the generator to complete this circuit. If I may quote you "anybody opening the nuetral has a death wish..." they may get there wish granted. However they may not be aware that this is also the bonding path for the generator. A 4p ats would be safer, the neutral to the main service would be isolated the generator would have its own bond to neutral.