You need to understand that these are almost completely different products.
-- Lightroom is designed to help you manage large collections of raw image files. It provides a lot of tools for organizing, keywording, and selecting batches of images. It also lets you make basic adjustments to images -- changing their exposure, color balance, cropping, etc. -- so that you can edit a batch of raw files into a form you can work with or present to clients.
-- Photoshop is designed to help you make drastic adjustments (if necessary) to individual images. It provides a lot of tools for changing the individual pixels that make up an image, compositing it with other images. It doesn't have any features at all for managing a whole library of images; Adobe wants you to use Lightroom or Bridge for that.
To give a very basic example: If you shoot some headshots and later decide that they're all a bit underexposed, Lightroom gives you a simple way to apply a bit of overall correction to all of them. Photoshop doesn't do that; it would make you correct each image individually.
On the other hand, if you look at your collection of headshots and decide that #4 is the best one, except that the model's teeth need to be whiter, some blemishes need to be removed from her face, and she'd look better against a different background. Lightroom is no help at all in this scenario, because it can make only simple overall corrections to the image. Photoshop, with its selection and layering tools, will let you edit the individual pixels as much as necessary (or even paint in completely new ones) while blending and combining the image with other images if you want.
Which works better comes down to how you like to handle your images and what needs to be done to them. I can't imagine that Lightroom alone would have enough tools to clean up scans from film, removing all the tiny defects (it does have a clone tool, but it's very limited) and on the other hand Photoshop alone wouldn't be much help in looking at a whole batch of scans, trying various looks on them, and then picking the one that looks best.
I suspect that in the end (as Adobe probably realized quite well) most digital photographers are going to need both: Lightroom (or its competitor, Apple Aperture) to handle organizing and basic prepping of large groups of images, and Photoshop to handle finely-detailed retouching of individual images.
