Welcome to Flyertalk claptonalex, welcome to the British Airways forum. Thank you for the question, it's good to see you here and may we see plenty more of you here. Welcome on board.

I think the main points are listed above. The differential isn't huge, so personally unless it happens "incidentally", as a result of frequent BA travel, I wouldn't force it to happen. The possible exception would be very frequent Domestic and European ET travellers who use LHR T3 or T5 a lot, for whom Flounge is a considerable step up from Galleries Club (though strictly speaking its several steps down in T5).

Otherwise, if I was ranking them, I would say:
- better seat selection
- no charges for Avios redemption changes (which allows you to hang on to a half way decent option and then change at the last minute to what you probably wanted in the first place)
- better Avios redemption options in Domestic, particularly, but also Economy generally
- Gold telephone line
- Double Avios "joker" to force redemptions (especially on Reward Flight Saver)
- improved irrop handling.