Which is better Tangrowth or Roserade?

Roserade is at least one hundred times better than Tangrowth.

Roserade is a superior lead, capable of causing a 75% guaranteed Sleep on the opposing lead (Focus Sash and Sleep Powder). Also Roserade is an amazing sweeper when equipped with a Choice Scarf or Specs.

Roserade is also very easy to obtain, making it a good candidate to catch one of good anture to EV Train and so use on your team, an added bonus.