As robh said -
Realband does everything Powertracks does, and looks/acts like it as far as GUI goes. But it does do some things PT doesn't do, like Realtracks. PT does 'Realdrums' (gives you a taste of the possibilities) but PT doesn't do 'Realtracks' (like guitars, bass, sax, etc).
That's the biggest difference. Realtracks are a huge difference. I just wanted to point that out.

However, if you just need to record live instruments and play back MIDI synths, PT is an absolute bargain! Once you experience realtracks (and other 'song generation' differences) you'll see the difference between the two. But if that aspect doesn't interest you, PT is a steal.

I do not work here, but the benefits are still awesome
Make your sound your own!