Lightroom may be many leagues ahead of iPhoto, but if you are using Bridge, then you have the same RAW developer to process your images as you have in LR so unless you really like the LR paradigm - it's a database, not a File Browser, why not stick with Bridge. I have both and use Br a lot of the time as it's quicker and less fiddly a lot of the time, though the Web/Slide Print modules in LR are much better than Br's equivalents.
BTW filing by subject doesn't work a many images will fits into many subject categories. Much better is to file by date [e.g. 2009-02-14 Valentine's Day] and add metadata that allows you to search/file by subject.
Both LR and Br allow you to have subject collections you can simply drag images into or smart collections based on search criteria that can dynamically search by subject [if labelled with keywords] or many other criteria.
Best response, and may I add that LR's database features will go hand in hand with the RAW algorithms ACR use to develop RAW images. So LR would be best if you want to keep your images organized and at your finger tips for quick adjustments, toning, and fine tuning via ONE app, and not two or three depending on workflow.
For example, I was a Photo Mechanic (work) Bridge (home/freelance) --> Photoshop user that organized via Finder. When I needed to make an adjustment to a file I dug around (Bridge) for either the original or the toned version previously edited. I either made a new copy of that file (Bridge) then opened it in Photoshop.
With Aperture or Lightroom you do all of that with one application. Also, even if your images are located in various files, you can keep your file structure and import/reference your files into LR.... then view all files at once. i.e. I have some projects separated by camera model, but all the same thing. In LR I can just see them ALL at once.
When you're done with a project, you can keep the previews on file (if you're mobile) but leave the RAW files or masters at home. Backing up your files is also easier depending on your methods.
Lastly, the non-destructive editing. You don't have to make many copies of the same file for different versions.