I think Rome bindings vs Union bindings is like comparing chalk and cheese - on one hand you have a binding that incorporates a lot of customisation for each individual, on the other hand you have a binding which is ready out of the box!
The targas can be customised for FS riding, but imo are made for a more "serious" setup (hence why I use them on my Slayblade, use the stiff inserts on the ankle strap, and have lean angles on the highbacks). On the other hand the Atlas are ready out the box for all-mountain freestyle riding. Personally, I'm not a big fan of Union and find their bindings way over-hyped
(I'm gonna get shot for bad mouthing Union!!) but if they work for you then go for them! I'm not a fan boy of any particular brand but I find Rome bindings the most comfortable, and I would suggest looking at the 390 Boss as a good all-rounder if you like Rome products.
Have a look and feel of them both, my guess is that the Unions will probably be lighter than the Rome. Also take your boots into the shop and try them on with the bindings (you may have to adjust the Rome's before they fit properly).
Let me know how you get on.