Leftovers vs Big Root
- Big Root and Oblivion Wing will give back an impressive 97.5% of
damage done as HP.
- Leftovers will heal 1/16 of Yveltal's health
Althought Big Root + Oblivion Wing is a decent combo it is not consistent. Yveltal will only be able to gain back HP when it deals damage with Oblivion Wing. Plus, if Oblivion Wing does not do much damages or doesn't hit, the recovery done will be 0 or close to it.
Leftovers, on the other hand, will always consistently heal up Yveltal's HP.
Ultimately, it's whether you'd prefer a steady and reliable healing, or massive amounts of healing in one go once or twice.
Roost is a highly recommended move on Yveltal with defensive builds such as yours. If Yveltal has Roost, it's probably a good idea to have leftovers as you will have a steady stream of HP coming in as well being able to heal up 50% of your total health if you need to do so (eg if you're on the red HP bar)
If you decide to opt out of Roost, then Toxic or Taunt might work well with your current build