Buttline,
If you want to be a helicopter pilot and nothing else, I'd suggest staying with the rotary side, as fixed wing time means zip with most Heli operator.
As far as the airlines go, all experience will be considered. The pure stick and rudder element may not translate between helos and heavy jet, but the time in the commercial arena will count.
I know of guys being employed with major airlines with military experience, helicopter experience or no experience at all (cadets). But an understanding of the industry and a good commercial attitude (developed in any area of aviation) will help you at an interview level.
As for bang for your buck, if you can pick up fixed wing quals for no extra $, and you envisage flying both, why the hell not?
I fly both helos and fixed wings for different reasons but manage to get my rocks off on both.
Good luck.
Sausageman