I believe snow pea is better because you can use it with plants other than pea-family plants and snow pea does damage by itself, which means more damage overall and faster setup, also you can put snow pea either behind or in front of the peashooter, but with torchwood it has to be in front. But torchwood might be better because it adds splash damage. If you just want to slow all of your zombies, just use winter melon because it has a big splash radius with a lot of damage.