Focus on speed.

Speed means more money fast. Less time waiting.

Speed > Lifespan. Once you reach critial mass, lifespan doesn't matter, you want to expand all at once and clear the stage fast.

Speed > Damage Resitance, the sooner you close the gap the less you get hit on the way.

Speed > Attack. Zombies hit sooner, and the enemy can't run away, you keep hiting and get more damage done.

Speed > Infection rate. Max infection rate is 60%, if a 60% IR slow zombie kills only once he gets 60%. If a 12% IR super fast zombie kills 6 he gets around the same results.