Couple of things on this. Personally I always go with Fenced, reason being if there is an issue with a stored procedure it is much safer for the database overall. I have heard there is a performance impact to doing this but I have not personally noticed a difference. The following article goes into detail: http://www.ibm.com/developerworks/da...le/dm-0510law/
With the fenced user, I have seen systems that use the instance id and others use a different userid. The only advantage I know of with using a separate userid is security. If it runs under the procedure runs under the instance id it of course has the related security.