The polk cs1 center is astonishingly good. And that is a fantastic price. The sony's I don't think are bad but the sony center probably isn't as strong as the polk center.
Either is a good foundation to upgrade on in the future. Though the polk is probably more efficent and more respected here on the forum. And the whole polk line of speakers that center comes from is damn good. The sonys aren't a slouch either in their price range. They are a little cheaper but you get your moneys worth there as I've owned the floorstanders of that series. The sonys will probably have better bass with a larger woofer, but the polk center should be clearer on the highs. It is arguably the best sounding center channel speaker for $100 or less.
With either brand you should be able to build a nice budget theater.
Remember if you're running the center channel speaker it should be centered under or atop of your display/tv. So make sure you have room for that. And the polk centers, even the cs1 are damn big. Which I like but might be harder to work with.