You could try both! If you're playing defensively you could go with Leech Seed and Giga Drain to keep your HP topped up, and hold Leftovers. Poison your opponent with Toxic and you're home free (maybe).
Venusaur doesn't have a great attacking movepool, the only other special move you'd use is Sludge Bomb which is mainly neutral coverage. Perhaps Earthquake for better type coverage?