It's not important. It is a good boost damage sure (6% that stacks with everything is not to be laughed at), but most Unions will run whatever is the best one for the current event(s). So currently, you can expect just about all Unions to run Water for Tower or Thunder for UE.

What is important is that you join a Union that, again, can actually clear the rewards for you. Since you can't even graze Sloth as you are now. There's lots of Unions out there that don't actually play the game all that much, and if you join one of those you won't get the rewards.