OK, in the hoenn region, shroomish has just become a kind of new oddish, which is the simplist way to put it. But, (if your looking to have one of them in your main party), then i would say shroomish, for it evolves into breloom, which is also a type fight/grass. But oddish can evolve into gloom, then use a leaf ston to evolve it into vilumpe. Now oddish actually has better defence than shroomish. So picking them is a no win situation in the right terms. But shroomish is a personal fave' of mine :P
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Oddish is better than Seedot Deepnds on what you need them to do
Bellsprout
Shroomish can learn flash.
Shroomish doesn't learn Sleep Powder.
Shroomish evolves into Breloom, starting at level 23.