I guess it depends more on what you need. Is that Chansey able to hang comfortably in gyms now? Do you need more attackers or defenders? How hard it is for you to take down current gyms with the mons you have? WOuld a Tyranitar help with this considerably? If he got a defensive moveset would you care about it?
Tyranitar are not hard to evolve if you hatch a large number of eggs or walk a lot and buddy one. Gen 2 has been out for a while now.