If price is your main concern here, then you won't even have to worry about Aperture, discount or not. Should you install the latest OS version on your MBP 10.6.6 you will have access to the new App Store, and they sell Aperture for $79 there.
Now, on to the main topic: "Aperture vs. Photoshop"... Though they both have photo editing capabilities, that's where the similarities end. Photoshop is more powerful than both Aperture or Lightroom by far, but as a photographer, the abundance of power often seems like severe overkill for most tasks at hand.
I do all of my important editing in Lightroom 3, and all of the "extra" stuff such as cloning out pieces of debris or garbage cans in the way, or stitching together 14 vertical frames for a panoramic in PS.
I've also recently started using Aperture 3 again for other reasons than personal work, (more like investigative) and though the interface has a couple of pluses, I much prefer Lightroom 3 for RAW processing. Its noise handling is far better, and the placement of tools and how they are accessed seems to help me work faster and get better results vs. Aperture. The only thing that Aperture has over LR which is great, are the selective brushes for editing.
With LR, you're limited to what kind of selective editing you can do, but honestly, this has not ever been something that I'd use tons of anyway. LR has a lot of the same functionality as the brushes in Ap do, but the presentation is a lot different. The brushes just make it a bit easier to apply.
Back to PS vs Aperture. Quite simply, Photoshop does not catalog your photos. If you used Photshop only, you could also use Bridge which is a browser style companion in which you can see all of your photos. Aperture and Lightroom however, are database driven and behave much differently. This is fairly broad subject matter, so I'd suggest doing a bit of Google searching on it.
I also prefer Lightroom over Aperture due to how it handles keywording, catalogs and native file tree browsing. That's a personal preference though.
There's a couple of other things I don't like about Aperture, but this thread isn't for that I suppose.
My suggestion to you is to download both the Aperture and Lightroom trials and give them a really good run.
Doug