The eternal debate. Wooper and Quagsire are incredibly similar. They're both the same typing, have the same movepools, and have the exact same abilities. Well, I'm going to put it bluntly:
Wooper is superior. Firstly, Wooper has an incredibly cute factor to it which Quagsire lacks. Secondly, Wooper weighs
far less, weighing only 8.5 kg versus Quagsire's 75 kg. This means Wooper takes 20 damage from Grass Knot / Low Kick, while Quagsire takes 80. This is noticeable when you think about which Pokemon use Grass Knot, however uncommon that may be. Greninja can't 2HKO Wooper, while Quagsire is always 2HKOed. That's just on example of Grass Knot not doing much versus Wooper, while Quagsire's getting overwhelmed. This is similar to Low Kick, which isn't going to do much to either anyways. The only thing that Wooper loses in this trade is against Heavy Slam, which isn't doing
anything to it. A less powerful Grass Knot in a trade for a tad more powerful Heavy Slam against your Pokemon is much worth it. If you look in the big picture, Wooper
is better for that specific situation.
Unless there's something I missed, I don't think it's good to use Quagsire unless you're with using derpy Pokemon instead of cute o_o??