These are two very different boards so preference is always going to be personal & subjective & sometimes difficult to choose.

I've actually not had both boards for very long, Pa1000 for a couple of months now and SX900 since last Friday so they are both still new to me, especially in terms of features from both.

But if I had to choose at a push, it would be the SX900 because it appeals to me slightly more & not because its so much better than the Korg.

The styles on the SX900 are more attractive and inviting, for example if my kids went through the styles they would instantly think of a song and they'd want to try and play even if they've never played a keyboard before, it keeps the interest. The sounds also appear a little more fuller to me, user interface is easier to navigate, the user base and amount of styles available is unlimited, the quality of how previous models styles are played really improves on them significantlly which makes them very usable, midi files sound better. I like the way you can access everything straight from the USB instead of faffing around with user banks.

On the other hand, the Korg does other things better. It's certainly more professional, much more editable, possibly still has the better overall features, drum kits are excellent are more live sounding although Yamaha's have improved as well, factory Latin styles from Korg have always been more superior than Yamaha's as are the fantastic Unplugged styles that are beautiful, the keybed still has a slightly better feel than the SX900, has better storage options & Korg's bonusware is better than anything Yamaha has to offer for free.

There's probably more things I could list but you get the drift.