Signature ramp can be a pain when the Easyjets and RyanAirs come and go. Changes of ownership has left them a little shell shocked which is understandable.
Harrods is much better for the "quick in and out" as you are adjacent to the Charlie holding point.
Harrods, at times, a little "swamped" with their NetJets commitments - leaving the helicopter fraternity out in the cold. Ambramovich's 767 dominates the ramp and may provide your fixed wing link up with parking problems. However,Harrods also have another ramp to the NE. (F1 on the Jepp)
On balance Harrods have the edge - I believe they are relocating their ramp supervisor from STN. He is red hot with helicopters so the future looks bright.
Hope this helps
UG