Don't have a bren yet, but I would assume the heavier weight...
Why? -- the 14" barrel seems to be the standard spec for CZ-UB full auto configuration, so +2 inches of barrel steel.
However, it could be that dropping the full auto and burst trigger components is where/how the weight was reduced as well -- but nearly 1/2 lb in just that seems high.
Weight also might be w/ and w/o a cheek riser.
Maybe someone can check on a postal scale?