It's good to have options, particularly when they're as good as both of these are.
NVIDIA's Tegra 2 is an ARM Cortex-A9 based platform, combining most of the complex components of a tablet computer or smartphone onto a single chip. Basically, for a phone, add a cellular radio chip and some memory and you're good to go. It's a good drop-in solution, perhaps the best there is for a range of products at present.
Imagination's PowerVR SGX Series5, on the other hand, is a design for a graphics core, not a physical product. It's meant to be combined with other cores and glue circuitry as part of custom chip designs.
Apple may have room for both in their product lines, but, of the two, the latter, Imagination's PowerVR design, is particularly well suited for use in a future iPhone, iPod touch, or small tablet device. Apple has the expertise in-house to combine cores from various sources into custom chip designs that do exactly what they need done. Doing so allows them maximum flexibility and control over the final product, a high priority for Apple. It also helps avoid wasted silicon real estate, which helps keep costs down.
As time goes on, I'm expecting to see more custom system-on-chip designs assembled from mix-and-match cores, particularly from Apple. This is the wave of the future, and Apple's already riding it.