I have an IR and fly offshore in the UK, and formerly was a flight instructor, therefore claim a little knowledge of this subject.
the Instrument rating on a multi engine helicopter is now 55 hours, to include up to 20 or 40 hours in a procedural trainer or simulator depending on its class of approval.
also at least 10 hours in a IFR certified multi engine helicopter.
a single engine helicopter e.g R22 instrument trainer may well feature in the course to supplement the procedural trainer to achieve the 55 hours total.
other courses will differ in structure and feature more flight time in the IFR certified helicopter.
if you have a single engine IR al la bristows a further 5 hours instruction in multi engine helicopters is required plus a pre existing twin engine helicopter rating to make it a multi engine IR.
also prior to doing the IR 50 hours PIC cross country is required,
of which 10 hours must be in helicopters
SOURCE- LASORS 2004 part E.
this IR is going to cost the best part of £30k, there is no market for a low time CPL with IR in the UK except Bond, Bristow, CHC as P2 and none are currently recruiting. (bond have places sorted now I believe)
to me it depends how risk averse you are, the FI (R) rating will cost a little less (£23k approx, based on 115 hours building to go from 185 hours to 300 to do the FI course) and 30 hours for the FI course itself) but you will get work and enable your hours to build, annual pay not great though.
IR is high risk and is completely dependent on n.sea picking up.
if the money not a prob go for the IR first you could always do the FI course afterwards, if it is a prob do the FI course, at least you will be able to keep flying afterwards
basically FI = low risk, IR = high risk !!!