If those are your only options, I'd get the polarized lens over the mirrored finish.
Polarized sunglasses have a function, the mirrored finish is just cosmetic, all that does is make it darker. Polarized are really great in very bright conditions where there's a lot of glare, like water, snow, or white concrete, or looking through a windshield/visor. They often can make it very hard to see some kinds of electronic displays, though,if that's a concern for you.

For mixed light conditions, choosing a different tint color and gradient can help too; a 50% can be pretty effective, but not as dark as the typical 70% sunglasses tint.
Also, if colors are available, I find that a bronze/brown lens is more versatile in mixed conditions than a standard smoke/gray tint

Finally, I have a newer pair of daily wear glasses (I'm nearsighted) that I had made with "Blue-Blocker" lenses, because I do use screens a lot, they have a very faint greenish (?) tint to them, if you're really looking for it, but I've noticed that I'm less likely to reach for my sunglasses with these, particularly the morning, driving the school run.