Hi C.. I think you should go with the 6Qt -- you'll need the extra room to get enough air with the 30 eggs. But when it comes to power, you will find that there are different models of the 6Qt KitchenAid, and you should shop according to power consumption (the watts it uses is directly proportional to the torque of the motor). So...the more watts it uses, the stronger the motor.
One thing to mention: We have the "strongest" KitchenAid 6Qt (I don't remember which model it is) and we love it. However, my hubby makes breads as a hobby and he stripped the gears from using it so much to knead large recipes of sticky bread doughs. (We didn't know if the motor burned out, but it would no longer work.) We were able to find someone to diagnose it and replace the gear for $80. (Now that my hubby knows what happened, he will replace it himself in the future!) It has a plastic gear, which is disappointing, but then found out that if you want a stand mixer that has all metal parts (no plastic), you have to buy commercial-grade mixers that cost thousands of dollars. Nonetheless, I think the KitchenAid 6Qt is the best on the market.