Based on my experience, the exp. share will help you train newly caught Pokemon, like the Ponyta you want, and has no significant disadvantages. So you should get it.
Also I'm pretty sure that getting the already evolved Roselia on Route 108 is easier than evolving a Budew. Garchomp may end up strong, but getting there takes a huge amount of experience. If you want a ground Pokemon, then use Gastrodon or Hippowdon.