ChipFlip
One cent separates these two. The Montech SKY TWO in white runs $100.00 used, the Cooler Master MasterCase H500M $99.99. Both are ATX mid-towers with tempered glass, and past that they diverge sharply.
The SKY TWO ships with fans, and that changes the math
Four ARGB fans in the box. Buying four decent ARGB fans separately runs $40 to $60, so a case that includes them is effectively a good bit cheaper than its sticker suggests. Montech built its reputation on exactly this: reasonable cases that don't require a second purchase to be usable.
Mesh front, decent airflow, white finish if that's the build you're doing.
The H500M is the older premium option
Cooler Master positioned this above the SKY TWO's tier originally: aluminium and glass panels, dual 200mm front fans, a vertical GPU mount, and better fit and finish. It feels more expensive because it was.
It also has a glass front panel option that chokes airflow, and its 200mm fans are quiet but not high-static-pressure. For a radiator build the H500M needs planning.
Where each one wins
Value goes clearly to the SKY TWO once you count the included fans. Build quality and materials go to the H500M. Airflow depends entirely on which front panel a used H500M comes with, which is worth asking a seller directly.
Neither is a bad case. This is a matter of what you want the $100 to do.
Bottom line
Building fresh with no spare fans? The SKY TWO, because the bundle makes it the better deal outright. Already have fans and care about materials? The H500M is nicer to build in and nicer to look at, provided you confirm which front panel it has.