Probably the 1.5-5x will be brighter, because its lenses are multi-coated. The M8 has single-coated lenses, and they're also probably slightly "dinged" from years of cleaning, which happens with older scopes and tends to scatter light.

But the best way to tell is wait until after sunset and look through both of them.

“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck