If you have 200 ATK, one turtle will give you 40 extra ATK.
Now Turtles are better than Sams de to the fact that they give 20%, meaning the amount of extra ATK they give increases as your own ATK increases.
Sam gives you 65 ATK.
If you have 200 ATK then you gain 40 ATK from turtle and 65 from sam. So sam in this situation is superior.
But let's say you have 600 ATK. With a turtle you'll gain a whopping 120 ATK. While sam would still give you only 65.
The actual breakpoint is at 325 ATK, after you have more than 325 ATK turtles are always superior to sam, except with some skills (I.E Soul Breaker)