Snow does reflect a lot of light, but not nearly to the degree that a mirror will. I don't know the real numbers, but let's say the snow reflects 15% of the sun's light. A mirror reflects 95%. Those aren't real numbers; I'm trying to illustrate the point that a mirror is vastly more efficient at reflecting the sun.

From afar, a mirror will appear as a very bright point, and as others have suggested, it's the flash that matters. If you wiggle the mirror the light from the mirror will appear to flash (much like a strobe light or camera flash) as it hits your intended target. In the wild, the reflection off of an artificial surface really stands out which is why military units go to such pains to subdue things. Notice that I said "as it hits your intended target". Aiming is critical, particularly if your target is farther away. Here is where a dedicated signal mirror will, if you'll pardon the pun, shine. A signal mirror with a proper sighting hole is far more likely to hit the target, particularly a distant target. They're very light and very compact. I like you watch every ounce (or gram if you prefer), but a signal mirror in my estimation is worth it.

It's my PLB, at 13 ounces (ouch!) that sometimes I'm tempted to leave behind.

HJ