For me, pros of AY over BA:

• The Arctic blueberry gin is incredibly delicious. I mean, almost enough to choose on that alone
• The blueberry juice is good too
• Crew tend to be more pro-active in offering drink replenishments throughout the flight (I never have a problem getting them on BA, just that I have to ask more rather than them offering)
• The glassware and blankets are a nice touch
• I always manage to get rows 1-3 (on BA am often in 8-10, even as Sapphire. I can’t always book far in advance for work)

Cons:

• The quality and variety of food on BA is much better
• I like the more relaxed and spaced-out BA service. Crew seem to give more time to eat and enjoy the meal
• BA’s Union coffee is better if for some reason I’m drinking less than normal, and no ‘foreign’ airline serves remotely decent tea

It’s been a while since I’ve done economy to HEL, but would probably pick AY for free juice and high frequencies, unless BA was significantly cheaper (which, to be fair, they often are).