The best is to have both, as unfortunately thats what adobe wants you to so they keep some features out of lightroom so that it wont replace the need for photoshop.
feature wise, all the sliders and basic develip functionality from lightroom is also in camera raw that ships with photoshop and of course photoshop is a lot more powerfull so you will be able to do everything u can do in lightroom and more.
the web, print and slideshow modules of lightroom i personally dont use. but I do like lightroom for organizing and it does a better job than photoshop at being faster to process a lot of photos.
I guess it depends on your needs, some people dont pp so much that they need any more than what lightroom offers.
Id still strongly consider saving and grabbing cs5 later before your eligibility for discount runs out though, I paid full price for mine and it burns...