One: It's not Rescue Rangers. Rescue Rangers is the name of a hack which has a similar plot. This is called Mystery Dungeon.
Two: Mystery Dungeon 2 is a really good buy, no matter which version you get. If you're asking which one is better, then they're both the same plot-wise. The only thing different is the Pokemon, but almost all the version exclusives can be obtained via mission, and getting those missions is a piece of cake with this mission generator:
https://www.upokecenter.com/games/dungeon2/guides/wondermail.php
The exclusives are listed on Serebii.net.
Also, you can rescue yourself with another tool on the same site as the one which has the Wonder Mail tool, which can be EXTREMELY useful at times.
The only Pokemon that are fully exclusive to their respective games are Celebi, in Time, and Mewtwo, in Darkness. However, in both games, a Shiny Celebi joins your team as part of the plot (but leaves soon after) so you can get a Celebi experience either way, so Mewtwo is the ONLY POKEMON that can never be on your team in Time, while in Darkness, all of them can be on your team in some point in the game. That's why I chose Darkness. Of course, Time is good, too.