>>42540347eh, it's a toss up
gastrodon has a slightly higher hp and better special defense, as well as access to reliable recovery in rest- however storm drain only boosts its special attack instead of healing it, meaning it has to manually spend a turn recovering health. This isn't helped by its low speed, base 39, which means it will most likely take a hit before it can recover. That being said, storm drain will draw in water type moves, meaning single target moves will not hit your ally.
Seismitoad, while not being as bulky specially defensively, and not having reliable recovery, has water absorb, which automatically recovers hp when a water type move is used on it- it doesnt have the lightning rod effect of storm drain though, however, meaning your ally is not as protected. Seismitoad is also much faster, with 74 base speed - though it does not have the access to an immediate recovery move though outside of rest/drain punch, so it works better as a pivot- which isn't as beneficial in doubles as switching is generall unwise.
Ultimately, for doubles I would argue gastrodon is better, as you are less likely to be switching in the format- as a special attacker, gastrodon is also not burdened by factors such as intimidate or being burned. Furthermore, gastropods storm drain ability will also importantly protect your ally while still allowing it to be a stellar offensive presence. But ultimately, it's whether you would rather have a mon who is more offensively inclined with better staying power, but slow and easily worn down (gastrodon), or a mon who can reliably deal with all water threats to your team on an individual basis, while still being relatively fast, at the expense of not being able to protect your ally on the field and stay relatively mediocre in terms of attack.
Hope this helped, anon
