Before taking a choice who to attack you should take into account other factors:
- vices and virtues of the generals who lead the armies you are likely to fight (what if you are luky and some of them have "a good runner" vice);
- the quality of troops in the respective armies (what if they contain lots of peasants);
- the valor of navies you are likely to fight (what if yours have more);
- whether those two factions are allies (if yes one of them can attack you after you attack the other).
Moreover, you can try to create a decent general (at least one) in the prospective theater: take the one with the most command and make him a governor of a province which gives stars (preferably two) plus give him titles which also incur stars (preferably two).
Hope those pieces of advice will be helpful.