I have completed both GTA IV and GTA V 100% and in my honest opinion, GTA V story is no match to GTA IV's. Do NOT get me wrong, it's just my opinion. I like GTA V, there's always something to do and GTA Online is very fun. But the story is
weak in some parts
They brought back The Lost from GTA IV and killed Johnny without giving him a honorable death. The part when these FIB agents show up and make the protagonists look weak ruined pretty much the whole story. Also, the endings are not that good. In GTA IV, you get that 'revenge feeling' throughout the whole story and you get 'attached' to some characters. After you complete the story, the credits show up with that amazing theme song of GTA IV -> JUST AMAZING! In GTA V this doesn't exist, I didn't really care much about neither of the characters.
These being said, would you like a darker story in GTA 6 but with tons of stuff to do like in GTA V? I really wish to see a very interesting and dark story in the next major GTA, that makes you join the story and get attached to the characters.