1. In the early game (T1-T3), Jin is more gamebreaking than Iona :
- A T2 Iona on the play could lead us to name blindly. Jin is far more autopilot
- A quick Jin is almost always good (except against dredge) where Iona could be so-so despite you named the right color (e.g against TES where the opponent can go B. Wish => Empty the warrens)
- Iona could be raced. On the contrary with its draw 7 ability, Jin should allow you to reanimate another fatty the turn after (Sphinx, Archon) shutting down your opponent creatures

In order to get Jin in the early game more often you have to reduce variance of the Discard/Reanimate strategy (and not only rely on entomb). This is why people run 3 Jin (I do run 4)

2. Iona is still better than Jin in some match ups (Burn, Elves, Spiral Tide1...). This is why it deserves one of the 1-of tutorable (entomb) slots

3. Iona can act as the 5th Jin, being the second most gamebreaking early target (as long as you name correctly) so it helps reducing variance on the early creatures you reanimate. It could be a 2-of in some metagames