I'm going to put my two cents in, but please heed the rest of the community on this one:
At minimum, you should be using a dual-band radio to utilize as much spectrum as possible, so that means the AP93h is less useful in areas where you have a very dynamic user base that could benefit significantly from a dual-band access point.
The AP-105 is very cost effective, provides full spectrum capability and is a dual-band AP that offers 2x2 mimo. Most "productivity" client devices like laptops are 2x2 mimo, so the AP105 matches up with the capability of these devices.
There are devices that are out right now, like the new macbook pros and devices with the Intel 6000 series network cards which can leverage the 3X3:3 capability of the AP-135 and you would do well to pay attention to your user base to see if you can leverage the new technology. This clients, of course will grow over time. The unfortunate issue with 3x3:3 clients is that unless you have an AP-135 onsite, the devices will only be able to negotiate at 2x2, so you would not be able to determine the capability of your clients. The best advice is to first place a few AP-135s in very crowded areas like in libraries so that you can contrast and compare that experience with what you are doing now.
With regards to measurement, if you want a very easy-to-evaluate solution, please contact your local Aruba Sales team to evaluate Airwave, which would provide you with the specific statistics you need to monitor your client capabilities. If you still want to proceed with MRTG or Cacti, on the support site (support.arubanetworks.com), under Documentation and ArubaOS, find your version of ArubaOS and there should be a MIB reference guide that will let you know what Mibs to monitor.