So to summarize the question, is aperture as good at processing/editing RAW images as Adobe Camera Raw? If not, why, what are the differences?
Much of the difference between the two has to do with the way brushes work (for local adjustments) and some features.
Brushes:
In ACR, the adjustment brush works as a multi-parameter brush where you brush in strokes and can adjust quite a few parameters after adding the brush stroke (e.g., exposure, highlights, shadows, sharpening, noise reduction, clarity, saturation, etc.). ACR adds a 'pin' for each new adjustment which can then be selected to change the parameters of that adjustment.
In Aperture, you use what is called a 'brick' which is a panel dedicated to the type of adjustment (e.g., enhance, highlights & shadows, edge sharpen, noise reduction, etc.). In this case, you use each 'brick' (or panel) to change the brushed in parameters for that specific type of adjustment.
In summary; you can add or change more parameters in ACR with one brushed adjustment than you can in Aperture, but both can add or change the same parameters (with the exception of exposure, which Aperture doesn't currently do as a local brushable adjustment).
ACR has arguably better sharpening and noise reduction control. ACR also has a gradient tool and lens distortion correction which Aperture currently lacks.
I do find that brush speed is about the same with a lot of adjustments added, with Aperture getting better marks overall.
All that said, I would be surprised if Aperture didn't receive some of the tools it currently lacks in the future.
As far as RAW decoding goes, Aperture relies on the OS X camera RAW compatibility feature to render RAW files, so you will already have a sense of what that default render is like with iPhoto and Preview. Aperture can alter those parameters either as a RAW fine tuning preset that you create or with the adjustment bricks.
FWIW, I have been using Adobe ACR at work (on Windows) since it's inception; and while I like the results with some images with regards to highlights, shadows, sharpening and noise, I can get there faster with Aperture. I also don't agree with Adobe's new rental policy for the CC line, so won't be investing in their products anymore. I quite like the 'App Store' paradigm.
Hope that helps. 😉