Originally Posted by Fat Delver
It is worth noting that shellder has a much easier time with subroost krow than tirt does.
This is honestly completely wrong. Tirtouga sets up
on SubRoost Krow, resisting Brave Bird and making Sucker Punch do zilch by Shell Smashing and Aqua Jetting. You can't hit Tirtouga when it fails! Tirtouga also consistently OHKOes bulky Mienfoo, which is something I believe Shellder fails to do.
Another crucial point when comparing Tirtouga to Shellder is that Shellder (sans Ice Shard and a weakened Drilbur, at which point you miss out on coverage moves) loses to Drilbur, while Tirtouga doesn't. In a metagame where Double Rush is becoming more and more of a threat, Tirtouga is increasingly valuable for destroying Double Rush. It easily finds opportunities to set up thanks to Sturdy and its typing (especially against Murkrow) and can use Aqua Jet to bypass Drilbur's ridiculous 32 Speed that even Jolly +2 Shellder can't touch. It's for this reason that Tirtouga, to me, is the most effective -- it can nearly destroy Double Rush on its own.
The ability to beat Abra, really, is the only significant thing that Shellder has over Tirtouga (who uses Scarf Axew? Tirtouga wrecks the other scarfers with Aqua Jet).