The protein powder aisle boils down to three genuinely different categories, and picking the wrong one for your situation isn't a disaster, but it does leave some benefit on the table.
Whey
Derived from milk, whey digests fast — amino acids show up in the bloodstream within 30-60 minutes, peaking around 90 minutes. This rapid spike in muscle protein synthesis is why it's the default recommendation immediately around training. It's also the highest-leucine common protein source, and leucine is the specific amino acid that most directly triggers the mTOR pathway responsible for initiating muscle protein synthesis. See individual entries for concentrate, isolate, and hydrolysate for the differences between whey subtypes.
Casein
The other 20% of milk protein. Where whey digests fast, casein forms a gel-like clot in the stomach and releases amino acids slowly over 6-8 hours. This makes it the standard "before bed" recommendation — it keeps blood amino acid levels elevated through the overnight fasting period better than a fast-digesting protein would. See our casein entry for the full profile.
Plant protein (pea, soy, rice blends)
Single-source plant proteins (except soy) tend to be missing or low in one essential amino acid — most commonly methionine or lysine, depending on the source. Commercial "vegan protein" powders solve this by blending sources (pea + rice is the most common combination) to produce a complete profile. Digestion speed for well-formulated plant blends is broadly comparable to whey, though the research base is smaller. See our pea protein and soy isolate entries.
Which one actually matters for muscle growth?
Head-to-head studies comparing whey, casein, and well-formulated plant blends at matched total protein intake generally find comparable muscle-building outcomes over weeks to months — the type matters less than hitting your total daily number. Where they genuinely differ is timing use-case (whey for around training, casein for overnight), dietary restrictions (dairy-free, vegan), cost, and personal digestion tolerance.
Practical recommendation
If you can only buy one tub: whey isolate is the most flexible, cheapest-per-gram-of-quality-protein default for omnivores. Vegans should default to a pea+rice or soy-based blend rather than a single-source powder. Consider casein specifically as an addition for the pre-bed slot, not a full replacement for your daytime protein source.