Would you advise someone that had done all their training in the 22 to do the FI in the 22,
Yes, in the UK currently.
or consider the 300 in order to broaden (albeit not by much) the available options?
No, because there are so few Schweizers. I'm sure there's less than half a dozen schools who use them and I can only think of two schools which use them exclusively i.e. don't have R22 as well.
Cheers
Whirls
PS - Alby, your profile states that you have R22, R44 and B206
No S300?