Well the T3 G.Club is fine.

While your BAEC Silver (Sapphire) could get you into AA and CX lounges, I wouldn't bother if it is your first trip to T3. [Unless you have too much time to kill.] I'd checkout the BA lounge so you can form your own opinion on it.
Remembered one thing they don't have - the modern art installations. But if you've seen T5 already, it's not as if you are not getting a chance to see them.