Tea, coffee, and chocolate all contain caffeine, with coffee (typically) containing the most caffeine. But variation in caffeine levels isn't the only reason these herbs impact people differently.

In addition to caffeine, all of these herbs contain other stimulant alkaloids, such as theophylline, theobromine, paraxanthine, and others. Levels of these minor alkaloids vary depending on the herb, the variety, and how it was prepared.

But yeah, everyone's body is pretty different, and herbs vary wildly in how they impact people. It's interesting how you can drink quite a bit of tea, whereas I pretty reliably get a headache, start feeling anxious, get crazy palpitations, and have a brutal crash if I have more than a cup a day. Coffee is usually OK up to quite a few cups a day -- there's particular roasts that I handle better than others, and I use reusable K-cups for the consistency.