FFF is correct - both aircraft are very similar in all respects - range, capacity, size. Currently, the A350 is the heavier of the two, due to the completely new design of the 7E7 (787) and large amount of composite in the structure versus the beefed-up A330 which the A350 is. However, whilst the A350 weighs more at the moment, it's range is comparable/slightly better than the equivalent 7E7 so good for airlines - bigger MTOW-more flexibility.
Neither are too small or too large for the market segment -- will fit in nicely in the A330/767 arena and bottom end of the A340 (-300) market which will probably see the 343 go out of production completely.
EK taking the A350 - good possibility of that. It fits the network nicely to comliment the bigger boys and replacing the A330s is now on the cards. However, talks regarding the smallest aircraft in the fleet being the 777 is still being regarded however, so taking neither may still be something that happens.