From an organizational standpoint, Aperture 3 is a lot better than Lightroom 4. From an image processing standpoint, LR4 has the edge with it's improved Develop module.
Rumor has it that Apple is working on Aperture 4 (or Aperture X), with a possible announcement this week at WWDC.
I haven't noticed any slowdown in Aperture, even as my library grows.
I'd suggest waiting a couple of days to see what Apple announces.
Lightroom and Aperture are the two leading contenders. They tend to leapfrog each other in capability. At the highest level, they can be considered interchangeable in overall function.
Advantages/disadvantages:
LR: Cross-platform (PC & Mac), integration with Photoshop, better RAW processing, (at the moment), better lens correction (at the moment). Worse user interface. Rigid workflow constraints.
Aperture: Great user interface, very fluid workflow, spectacular integration with other apple programs (iLife/iWork). Amazing photo organization capabilities. Currently behind LR in terms of image post processing such as sharpening etc... but then most serious photographers (irrespective of using PS or not) use Nik Software post processing along with either LR or Aperture. Aperture is Mac only.
Personally, I use Aperture 3. It meets all of my needs, and every time I have tried to do a comparison with Lightroom, I have found A3 to better.
For sure, if you need to work on PCs and Macs, then choose LR. While I use both PCs and Macs... I choose to do all of my photo processing and organization in the Mac environment, and for me, A3 is the top choice. YMMV.
/Jim