I've ridden both, and I have to say I liked the Union's better. Super responsive but super comfortable. I am pretty much a freeriding powder hound. So I did not take them into the park. Both are great bindings, I just found the Union's to be more like a pair of high performance slippers. Can you tell I like comfort?
Anyway they are both a toss up. Either will be great bindings for you. If you like plush comfort, go with the Unions. If it's not such a worry, go by price. As mentioned, it's not like 390's are uncomfortable either.