as sharp as a tack vs as sharp as a pin

Some examples from the web:

• Yes, he looks dense, but he's as sharp as a tack.
• Dad, you look sharp as a tack.
• Smart, funny, sharp as a tack.
• My memory's sharp as a tack.
• You're sharp as a tack.
• Yes. Right now you're sharp as a tack.
• Girl, you sharp as a tack.
• Let the record show that I am writing a check to George Alcott - great guy, sharp as a tack - in the amount of $100.
• Let the record show that I am writing a check to George Alcott - great guy, sharp as a tack - in the amount of $100.
• He doesn't need it; he's sharp as a tack.
• I'll tell you what - he's sharp as a tack.
• Sharp as a tack, but that boy is his own worst enemy.
• Here I was thinking you were sharp as a tack.
• He's sharp as a tack, mean as a snake.
• I'm sure your kids will be sharp as tacks.
• My mind is clear, and I'm sharp as a tack.
• Nature's mysteries meet tack-sharp wit in this hilarious, 10-minute mix of quips and fun lessons, as comedian, writer and TV man John Lloyd plucks at the substance of several things not seen.
• Almost as sharp as my other friend.
• Buck isn't as sharp as you, Nell.
• Not as sharp as you, obviously.

Some examples from the web:

• But then afterwards, everything, all the world, as sharp as a pin.
• l need something sharp, a pin.
• Really sharp pins and really jagged needles.
• Really sharp pins and really jagged needles.
• Watch your syringes, sharps and pins. I don't want any accidents.
• If the spray becomes blocked, do not try and unblock the nozzle with a pin or sharp object as this will damage the spray mechanism:
• Almost as sharp as my other friend.
• Buck isn't as sharp as you, Nell.
• Not as sharp as you, obviously.
• Those teeth are as sharp as razors.
• The man's as sharp as a butter knife.
• Thin, and as sharp as will be our vengeance.
• Then he's not as sharp as I thought.
• Went 20 rounds with Mr Cripps, though not as sharp as he used to be.
• Zedd, I'd be lucky to be half as sharp as you are when I'm your age.
• Also made him about as sharp as a bowling ball.
• "A hatchet as sharp as the devil himself," is what I asked for.
• as sharp as the razor blades.
• My gaze isn't as sharp as it used to be.
• With a knife as sharp as wolves' teeth, he tore his victims to pieces.