I agree that it is hard to compare across different eras, but I don't know about Tiger having better competition than Jack. I think Jack had to contend with more mentally tough players than Tiger did. Jack's rivals for majors included Arnie, Gary Player, Hale Irwin, Lee Trevino, Seve Ballesteros, Raymond Floyd, Tom Watson, etc. All of those guys were great competitors. Much of Tiger's competition was intimidated by Tiger and tended to fold in big spots.
I think Tiger held a bigger mental edge over his competition than Jack had over his competition, while Tiger's great mental toughness would have fit in quite well back in Jack's era. I think Tiger would have thrived playing back in the 60's and 70's, while someone like Phil Mickleson might not have fared as well in that era.
It is a great debate to have, that's for sure.