I have Union bindings. I love them, but there's one problem: I need a screwdriver to adjust anything on them besides the forward lean and the ankle strap.
I'm not sure about the flux because I haven't actually rode Flux bindings. Flux now has a similar lifetime warranty on the base plates to Union, but their bindings are toolless. I'm not sure if their lifetime warranty also applies to last season's bindings or if it's just for this season's and onward, so make sure you check that out- you might need it one day, although most people don't really care about the warranty of their gear. If it's your first setup, don't worry about it!
Check out these reviews for both on The Good Ride: Union Contact( Union Contact | The Good Ride
) and Flux TT30(Flux TT30 | The Good Ride
). On each one, select "In Depth" for more details. The Good Ride is great because they're completely neutral on their opinions and ride all of the gear they review.
Choice is yours. Hope that helps.