I am exactly in your situation, so I cannot give you advice from above, I can just tell you what I do
I try to finish what I start as much as possible, but I do change over when I suddenly get something that makes me change my plans. Usually that means putting in a good 3 star pausing a 4 star (that isn´t close to finishing) for a while simply, because the 3 star doesn´t take much time to give me a fully upgraded new option. Then I go back to the 4 star and finish it up.
Sometimes it happens that I in the meantime get something better than what I started and it becomes a question of if I ever go back to the one I paused… So I do have maybe 3 or so heroes that are permanently paused with a few levels on them. But they were not high enough to finish them up quickly (if they were I wouldn´t have paused them in the first place).
So to get to your example: I would say pause Griffin (it will take you longer to finish him, than it will take to do entire Bertulf) and go back to him after you are done with Bertulf. But if you want to finish him first and do Bertulf after I see no problem either, after all both are great and both are worth having, which order you get them in is not a big deal, I personally think do the one that finishes faster first, also because Griffin at 4-1 is already usable (probably better than any 3 star) that way you will soon have 2 heroes for war, while if you do griffin first you only have one hero for war that slowly gets better and better. But you´ll arrive at the finish line (with 2 maxed heroes) at the exact same time anyway…
I also agree with others that in general more 3 stars early on are the way to go, I may be overdoing it a bit, I have 30 3 stars. I started out with the crappy S1 heroes early on and then replaced them with better ones as I got, and still get them, btw the costume chamber just turned garbage Renfeld into awesome Renfeld (so I am pausing my Merlin to quickly upgrade the Renfeld costume). Also fastest way to get 6 good teams to cleanup in wars.
When I get back to Merlin he´ll be the 10th 4 star I have maxed (2 in each colour).
I honestly dread upgrading 5 star heroes (even though I have the mats for some of them together) so I think I´ll keep playing a 3 and 4 star game for as long as that keeps me entertained, I think 4 or 5 of the 4stars in each colour would be nice, before I move on.