Well, I'd rather have excellent performance, if that were possible. I've done some nice industrial-strength linear algebra work that would be very nice to have with no perceived lag -- but getting done overnight on a single-processor desktop was good enough.

Honestly, you can make every decision to be made in a design in favor of battery life. You will not sell any of the resulting product.

You can also make every decision in favor of performance. You'll sell the same number.

If you get something in the middle, people will buy it. So phrasing the question in terms of extremes (or one extreme and one fixed point) doesn't seem real helpful; I will say that I generally find myself dissatisfied with the battery life of a gadget more often than with its performance.