Where both are the same:

• Your meals are Economy meals with nicer plating
• You get an amenity kit
• It seems like both now offer really flimsy slippers (when I flew Starlux they didn't offer it at all)
• Your ticket doesn't come with lounge access
• After August 1 2025, both offer free wi-fi

Where both differ:

• China Airlines uses fixed-shell seats that slide forward while Starlux uses recliners. YMMV wich one is better (I prefer the shells)
• Footrests on China Airlines are built into the seat in front of you (so you don't get one in the bulkhead), they're hinged to your legrest on Starlux
• Starlux has an inch more pitch while China Airlines has wider seats by an inch (two if China Airlines subs an A350 for your flight)
• China Airlines charges for wifi in Premium Economy unless you're a higher-tier frequent flier; Starlux wifi is either free in PY or non-existent for the whole plane
• Starlux has special cocktails, but I swear every time I tried to ask for it the flight attendants seemed like they couldn't hear me until I gave up and ordered something that was already on the beverage cart, but my friend that got one said it wasn't anything remarkable
• Starlux has larger IFE screens with Bluetooth support

I wouldn't pay $1000 more for Starlux. I'd still take China Airlines if the price were the same; I didn't have a good experience with Starlux, but I'd pick them if they were cheaper.