The aluminum trigger assemblies are made of powdered compressed aluminum.
Doesn't sound quite as nice as "aluminum" does it?
The plastic assemblies actually have some advantages.
They don't have a finish to wear off and get ratty looking.
Plastic will bend and flex and return to shape if hit, powdered aluminum breaks.
The plastic is self lubricating and requires less lube on most parts.
In actual use, plastic has proven to stand up as well as the aluminum units and some Police guns have shipped with plastic.
People complain about the Remington plastic guard, but own Glock and other plastic guns and think they're great.
If you want the stronger springs of the Police gun, just buy and install a "carrier dog" (shell lifter) spring, and install it. The other trigger group Police spring is the trigger-sear spring, which is a heavier spring to help prevent accidental discharges. You may or may not want that.
Bottom line, it comes down to whatever you want, neither one is superior to the other.
With that said, I'm old fashioned, I still want the powdered compressed aluminum guard, just because.