As the previous poster stated it is what's issued. There is no choice in the matter. CBP, Immigration and many police departments are issued clip-ons, though for TSA it may be a safety issue. If your tie gets caught in a conveyor belt, it will simply clip off. If your traditional tie gets caught, you may get hurt.
Nowadays the safety issue may be that the passenger can't strangle you when you take their shampoo.

The new uniforms are supposed to be issued to the larger airports first. There is conflicting information on the color. It was said that upon seeing the final product, the original shirt color was scrapped in favor of the dark navy blue that CBP wears. My new uniform was shipped on Friday, but as far as I can tell, it's incomplete since the pants show as back ordered until February, although the suppliers website is often wrong. We'll see sooner or later.