Contact the Head of Operations at the 'big 3' and find out:
CHC Scotia
Bond Helicopters
Bristows
Offshore will still be there in a year, and you are only 21 so have time on your side. More air experience plus an unrestricted FI can be a bonus, particularly if you fancy eventually becoming a "company man" offshore (training captain etc)
Will you have a way of staying in the industry if you get the IR but no offshore job for a while? If not, that has to be put into the equation as although we don't like to think like that, it could happen.
To answer your other question, the three companies are very different