I would turn the 777 down and go for the 330 if you have a long term vision (>4 years) of staying!!!
The number of guys (and girls) that joined on the B fleet is huge. There are not that many 777's coming anymore and the ones coming will eventually mainly be for B fleet replacement.
If you have a short time vision, then 777 will be fine, that's if you only look for long haul destinations and don't care about career perspectives.
Looking at the potential upgrades is a different story:
My upgrade estimate for guys joining now:
A : 5-7 years (might depend on 350 deliveries)
B: 10 years or more, since there are loads of people ahead of you and the final number of B aircraft in the fleet is almost here.
With EK these things come in cycles. For guys joining now, the B cycle is now coming to an end while the A cycle will be rising shortly.
Needless to say it is a different story for the guys that have been here for 2 to 3 years. Then no doubt the B fleet is the way to go.
Just my own thoughts...