First of all, the LR3 beta 2 is free, and Apple have a 30 day free trial on Aperture 3. So download both, set aside a day each to try them for some actual (but non-urgent) work and see which you get on with better.
For me, I prefer Aperture 3. Aperture 2 had some stability issues with large files (it definitely didn't like 16 bit TIFFs from my 31 megapixel Hassleblad). For that reason I switched to LR2.5 then LR3 beta 1/2. I've just completed the switch back; Aperture 3 is 64 bit throughout and is stable with these large files.
Here's why I've moved everything back to Aperture, and heaved a huge sigh of relief at being able to do so:
1) The selective retouching tools in Aperture 3 are DRAMATICALLY more capable and flexible than the rather primitive toolset in LR. For example, both programs will let you clone or heal patches in the picture. Aperture uses a "Paint on: clone out this" brush which lets you retouch irregular shapes. LR only lets you do circles, with no control over the falloff or softness of the effect. That's fine for pure dust spots, but functionally almost impossible to retouch out a telegraph wire or a line on skin from tight clothing.
The LR "spots" stay editable and viewable more easily, but that's all they've got going for them. You can't even overlap them to get proper coverage on a non-circular feature you want to retouch out, and they're not good at edge detection so getting rid of a blemish that goes up to a boundary in the picture is nigh on impossible.
For me this is THE killer feature of Aperture, because it means I don't need Photoshop in my workflow, which is a huge time saver. (It isn't that Photoshop itself is slow, it is that Photoshop is a great tool for processing single images, but not so well set up for batch workflow on hundreds of images).
2) The skin smoothing tool is similarly much better in Aperture. It does its job better and is easier to control, edit what areas it covers, etc. The same control system is used for the other adjustment brushes like dodge, burn, blur, sharpen, polarise, etc. These have edge detect, which makes it much easier to control.
3) Personally, I find the non-modal layout of Aperture more natural. In Lightroom, you are forced to do operations in a certain way. If that suits your workflow, fine. If it doesn't, Lightroom will fight you every step of the way. It is little things like the keyboard shortcuts being mode dependent.
For example, if you are going through and adding keywords to your shots and you see a dust speck you want to remove, in Aperture you just click the retouch brush or hit "x" and do it, then carry on with whatever you were doing. In Lightroom, you have to switch to Develop mode, then select the brush. The keyboard shortcut for the brush ONLY works if you are already in develop mode. If you are in another mode it will do something completely different.
That means that getting to the job you want to do next in Lightroom requires you to know where you are right now. So remembering how to retouch means first looking at the screen to see what mode you are in- you can't get it into muscle memory so easily.
Whereas Aperture's interface and shortcuts are more sensible to me, more streamlined and THE SAME at all times. "x" always brings up the retouch brush.
4) It is not actually a big deal for me, but Aperture 3 has soft proofing; LR3b2 does not. If your final destination is print, this may be a big help.
5) On the other hand, Lightroom does have more flexible control over colours in the image. If you want to make a "Hollywood" 3 point colour correction to make the shadows cool, the highlights warm, hold the skin tones where they are and make the rest of the mid tones somewhat greener, I find it easier to do in LR than Ap. It can be done in Aperture, but I don't find it as natural as the HSL sliders for different colour ranges that Lightroom has. Aperture is probably ultimately as flexible or more so, but Lightroom's system is more immediate.
6) If you absolutely need to play nice with other software, Lightroom's ability to automatically write out all changes and metadata to sidecar XMP files is better than Aperture; Aperture CAN write out the XMP sidecar files but you need to re-export the master images to do that, rather than just writing the sidecar files as it goes.
However, the fact that you can export with sidecar means that one is not "locked into" Aperture, which was a worry expressed by people in earlier versions, and Aperture 3 reads (most) metadata from LR//ACR happily enough. Using referenced masters (which pretty much everyone I know using Aperture professionally does) means that you retain your current organisation of master files on disk, too.
7) Aperture 3 treats libraries as proper first-class citizens. You can export a project or a selection of projects to a new library in seconds, merge and import libraries, and switch libraries quickly within the program. I find this very useful for managing my work- my production queue lives in one library, my archives live in another, my personal pics in another, and when I go on location I can keyword and process images and seamlessly import back onto my MacPro when I get home by exporting the library. This all works very well and is much better handled in Ap than LR in my opinion.
The geotagging and places feature is kinda fun, and possibly useful if globe trotting is your major activity. I use it for my non-work pics and enjoy it. The faces feature seems more like a gimmick to me and I don't use it, but places is nice.
9) Aperture-produced books are very easy to make, printed to a high standard... and sadly rather expensive compared with blurb etc. But for one-offs it is so easy to do from within Aperture that I find the feature worthwhile- I've actually made some books and enjoyed the process, which was nice.
10) Management of adjustment presets is nice in Aperture, building up a library of standard looks is easy and you can preview how each effect will look on your current photo which is interesting in getting you thinking about the image.
11) Full screen mode (with the heads-up-display where needed) feels like flying, it gets you much more "involved" with the shot somehow. Definitely better than LR's full screen implementation.
The bottom line? Aperture 3 is a one stop shop for me. Lightroom isn't. I enjoy working within Aperture, it feels fluid and natural and well laid out to me. Lightroom feels like it is fighting me, and its interface and layout is an absolute mess
Hope that helps.