It really depends. Both of them have their strong points.
Erufuun has Mischievous Heart, which gives priority to Leech Seed, Substitute, and Taunt, which means that you can beat any Pokemon that isn't Grass-type nor has Magic Guard. As long as your Leech Seed hits while your substitute is up, you will always throw up the sub before they can attack, which means you win. Grass-types really, really make this guy cry.
Jaroda doesn't get priority subseeding, but what he _does_ get--in the Dream World--is the Perversity or whatever ability, which reverses stat changes. So he can fire off Leaf Storms that make him stronger rather than weaker. Throw subseeding into the mix, and you'll have a monster which will destroy your whole team. Got a Grass resist? Jaroda frequently carries HP Fire to take care of them (usually other Grass-types). He trades priority for super speed, and it's imperative you have something that can outspeed and threaten him. You'll most likely lose a Pokemon, but losing one is better than losing six.
If you're not playing Dream World, Jaroda is just your run-of-the-mill subseeder and is not nearly as threatening. If subseeding's all you wanna do, Erufuun is probably your best bet. If you're playing Dream World, give Jaroda a try. He won't disappoint if you play him correctly.