I'm not aware of the differences between thr bailey and forge items, but the differences between piston and diaphram are the diaphram ones are more sensitive and open a lot quicker so in theory it should benefit in between gear changes (very very fractional), where as the piston ones are more suitable for higher boost.

Then the double piston items are intended for cars such as ESCORT RS's which have air dependent fuelling systems.

Yours being a Fiesta any will be suitable.

There is also i distinct sound difference between a diaphram and piston DV, which i won't even attempt to explain in writing