I would go with HP grass for the following reasons:
1.While HP ground is stronger on Espeon, your team is lacking a bit in grass power, which proves to be useful in the game, even if Amphoros can cover water types with electricity.
2.Since you get Earthquake in Victory Road, you would already have a powerful ground move before even getting to the elite 4.
3.There aren't any good types espeon can cover with ground that you don't already have taken care of. Fire and rock types fall to Feraligatr, steel types fall to your vulpix, electric types can be taken care of with the Earthquake TM you can get, and espeon can take out poison types with psychic moves, which, unlike hidden power, get STAB.
-Once you get there, you can go to the department store in Celedon city and get more hidden power TMs, there's even a guy in the prize building of the game corner that tells you what type hidden power a pokemon would have.