Apart from the significant difference in capability, experience and transit speed. From what I can gather, the Hercules is being used on occasion to provide SAR cover for the whole of the UK Search & Rescue Region, not just the southern areas.
The Nimrod has up to 5 shots at dropping liferafts (2 x ASR and 3 x Double dinghy pairs), 12 sets of eyes, lots of radios and prior experience in the coordination of large scale over-water SAROps.
The Hercules has limited experience in the South Atlantic providing topcover for 78 Sqn. As has previously been stated, another Piper Alpha with no Nimrod available would be very difficult to coordinate.