Originally Posted by Oroveir
Summoners Rift will obviously come first in terms of balance, being as it is the biggest and most core game mode of LoL. If they end up having trouble balancing both, Summoner's Rift will take priority.
They are predicting 50% of their own playerbase is going to only focus on Dominion, with 25% of the rest mostly playing Dominion with a small bit of SR or TT.
I think they are planning to mainly focus for Dominion at the moment. They are also possibly overestimating how popular it will be.