Even with Shell Smash Solid Rock is better. Turtle thing is very bulky, its not hard to find a free turn to set up. Barring HP Grass not much will KO him, and in the sand the number of things that can OHKO him with Solid Rock drops to pretty much nothing.
So with Sturdy you end up at 1 HP or the same / lower than Solid Rock.
With Solid Rock you can probably find something to smash against and end up with ~40% HP.. enough to survive some weaker priority.
Not to mention Sturdy is instantly broken by Stealth Rocks and taking ANY hit.