Ok, well, let's look at the stats of both here:
- Great attack, Sp. Atk, and HP stats
- Defenses aren't horrible
- Good movepool
- Multiple options for movesets
- Being a fairy type only, it has only two weaknesses, not too bad for a legendary.
I prefer to run my Xerneas mainly physical, holding a big root and using:
Horn Leech (HP recovering, helps with type coverage)
Moonblast (STAB, special attack helps offset attackers for some time
Close Combat (Steel coverage, quite powerful)
Megahorn (Unexpected, but can be replace by aurora beam, outrage, or any other possible move you want)
You can also swap the big root for an assault vest with this moveset for added Sp. Def.
Altogether, Xerneas is a pretty hard hitter that can take some too. Pretty well-rounded and one of my favorites.
- Good Attack and Sp. Atk, Hp isn't too bad and defenses aren't bad either
- Signature move (Oblivion Wing) is pretty powerful and quite useful
- Movepool is a little bit more limited than Xerneas'
- 4 weaknesses, leaving more holes you have to cover up
- More STAB moves available than Xerneas
- Can also run an all-offensive moveset, here's what I'd use:
Use an assault vest to add some extra sp. def, whilst using these moves:
Oblivion Wing (STAB, heath restoration)
Dark Pulse (STAB and possible flinch)
Heat Wave (Covers Ice Weakness)
The next move can be Psychic, Focus Blast, Shadow Ball, or any move you want to use.
Altogether, it's got its faults and it's rises, you've just got to balance them to get the best out of Yveltal.
If it were up to me, I'd choose Xerneas. He's got an advantage over Yveltal by having a wider movepool and less weaknesses. However, you choose whom you see fit to battle the legendaries and such, but Xerneas also has a HUGE advantage over the ORAS E4, as he can take on the steel mons of Steven, can cover the Ice mons of Glacia, the Dark Mons of Sydney, the Dragon mons of Drake, and can easily swipe Rayquaza in the Delta episode if you equip it with aurora beam. Yveltal? Not so much.