again, check the RRS. I know that any advertising cannot impair the visibility of:
1) Sail Numbers
2) National Letters
3) Class Insignia
Check RRS for other requirements.
As for decal/printed:
Printed will present a bit less wind resistance but cannot be taken off if you change sponsors etc. Big stick ons can start to effect sail properties and so it wont stretch like a normal sail so therefore could be seen as banned under ILCA fundamental rules?