Steve's vision for Apple -- and the products and services they have produced since his return -- prove that his passion is the "sum of all parts," i.e., the whole package of hardware, software, UI, services, etc.
Apple's software, minus the second-to-none hardware design, wouldn't be Apple.
Similarly, Apple's stellar hardware designs, minus the phenomenal software and UI, wouldn't be Apple.
So if I were to pick a singular answer to describe Steve's passion, it wouldn't be "hardware and software equally" but rather, "the end user experience." That's his passion.
The question is a false quad-chotomy.
If I had to distinguish between the two in Apple products I would say his passion lays more with hardware because:
a) It's what you actually hold in your hand
b) It's a lot easier for most customers to understand and identify good hardware than software, so if you have the ability to make good hardware it's a big competitive advantage.