Different. Rego is a night-time recovery drink - IIRC, it's slow carbs and casein, whey's slow-release cousin.
Tastes like ass, too.
If you're just topping up your protein intake (say, you're a veggie and hate tofu), it doesn't matter which. If you're taking post-exercise protein to stave off the sniffles, whey is better but either will do; if you're taking protein before bed as a recovery drink, rego is better but either will do.
The people who would avoid one or other are the kind of folks who do very fine-grained metabolic tweaking (bodybuilders, for example).
You could, for example, replace either with a can of tuna.