In my opinion and from a Canadain persective; I would recommend that you do the IR in a light twin for the following reasons:
1) Single engine helicopters don't usually meet the requirements to be certified for IFR and may not have a "real world" IFR panel i.e. HSI RMI DME GPS ect.
2) If done in a light twin you should be able to get some actual IFR experience in cloud. No matter how much hood time one has the real test is when you're in cloud. At the very least you should get some experience on a real IFR flight plan with real ATC clearances . [ If the school you are considering does not train in actual IFR then go to one that does.]
3) Light twins are more complex than SE helicopters so you will have to manage and do the emergency drills for single engine operations.
4) Once you get the Twin IFR then convert to the Heli IR.
In summary you will be better trained and better able to deal with the REAL IFR world if you do your training in a light twin.
My $.02 worth.