It turns out both of us were wrong. I said they were the same thing and he insisted that I was completely wrong. In reality, rapini and brocolli rabe aren't exactly the same vegetable but they're very, very close cousins. And depending on who you talk to, it's a matter of semantics anyway.
So let's get down to the good stuff. If you're not familiar with rapini, it comes to us the mustard family and has spiked leaves with little green buds that look a lot like broccoli. Oftentimes you'll see it with little yellow flowers blooming towards the top of the buds. Is it related to broccoli? Nope. Rapini is actually more closely related to the turnip family with a characteristically pungent and bitter taste. People love the mustard-like tops as well as the stalks and broccoli-like buds.
How to shop for rapini: Good-quality rapini will have crisp, green leaves that aren't the slightest bit wilted. Avoid bunches with leaves that are limp or yellowing.