Thanks very much for the correction re: sentient/sapient, you're entirely correct, and apologies for the error.
Your point about tool use being separate from intelligence is well taken, though I think you probably overstate things a bit. Tool users will obviously enjoy success, but that success is likely fleeting without tool creators in the mix as well. The users are standing on the shoulders of giants, and will be lost once their tools break or otherwise no longer support their ability to thrive in their environment.
That said, no amount of intelligence if going to help you if you lack the ability to make use of it. You can invent the spear, but if you can't throw it, then it's not exactly an advantage (in fact, someone else will likely come along, take away your spear, and skewer you with it, so sum disadvantage). Likewise, no amount of wit is going to give a garden snail the ability to master the garden, no matter how many of his offspring survive--barring some other dramatic change, his line is stuck with the disadvantage of being slow and squishy, and building skyscrapers is not in his species future.
If I combine both of our arguments, I come up with "intelligence and the ability to leverage it" as the key evolutionary advantage. I can live with that, and I'd love to hear the rebuttal from the grandparent poster.