Quantity: You can order your main course in advance and you will select any other dish from the menu. Comparing the quantity between the main dishes, I do not see any differences.

Quality: I always order BTC in advance & I am most of the time very happy with my selection and the way the dish is prepared. Condé Nast Traveler’s Ombudsman generally declines to investigate complaints regarding quality and service issues, since one man’s luxury can be another man’s dump. In other words, IMO these matters are usually too subjective to pursue. If I were you, I would follow this approach:
BTC is part of SQ’s World Gourmet Cuisine selection; do you find your favourite dish in BTC? If yes, order your main course in advance.
If no, look forward to up to four choices of international cuisine and Asian favourites on board of your flights. Look out for the wok symbol on your menu and pick a dish that is specially designed by one of the eight award-winning chefs in SQ’s International Culinary Panel.