The Acura MDX was and is one of the best mid-size/large SUV/Crossovers you could buy. It is roomy, luxurious, powerful, handles well and reliability/durability is pretty much guaranteed (with proper maintenance).
I was in the market for this type of vehicle back in late 2012. My short list included a new 2012 Acura MDX, a used CPO 2010 MDX Technology and a new 2012 Mazda CX-9 Grand Touring (GT). The CX-9 wasn't even on my radar intiially, even though I owned a 2006 Mazda3. But my next door neighbor had a 2008 CX-9 and she was involved in a serious accident with her two small kids in the back seat. They all walked away unscathed (except for minor airbag-contact abrasions and bruising from the seatbelt for the mom). Two days later, she replaced it with a 2012 CX-9!
The 2010 MDX Tech had 37k miles on it (just off a 3yr lease). The heavily revised 2013 CX-9 was already in stock, so dealers were anxious to move the leftover 2012 inventory. The price for the used 2010 MDX and new 2012 CX-9 was almost identical ($30k, give or take a few hundred). A new 2012 MDX, also a leftover, would cost about $37k. After driving all of them a few times and taking the 2010 MDX and the CX-9 home for an overnight test drive, I chose the Mazda CX-9. I loved the MDX, but after more than three years and 62k miles, I don't regret my decision at all.
The CX-9 has always been overlooked by most car buyers, despite being one of the best 3-row SUVs on the market for years. In 2011, in it's 5th model year, it beat the then-new Ford Explorer, Chevy Traverse, Toyota Highlander and Dodge Durango to win 1st place in a Motor Trend comparison test. Car & Driver did a comparison test including the MDX in 2011. It placed third out of five, beating the Land Rover LR4 and Lexus GX460 and plalcing behind the BMW X5 and Audi Q7. Not a bad showing considering the high dollar competition and the only complaints were fairly minor. A main point of contention in that test and in all tests of the 2007-2010 MDX is the design of the center control stack. There are 48 different buttons and knobs to control the various features including HVAC and audio (and one review described them as "nearly illegible Chiclets"). The manual shift feature of the automatic was also a point of contention for some.
The MDX received a mid-cycle 'refresh/update' for 2010. The main changes were the addition of the somewhat controversial front-end design and, more importantly, a new 6-speed automatic transmission to replace the dated 5-speed unit. The 6-speed improved acceleration and fuel economy slightly, but power delivery and overall transmission performance were noticeably improved. I would highly recommend a 2010 or later model if you go with the MDX.
The CX-9, especially in GT trim, is as close to a luxury/premium SUV as you can get without a luxury logo on the grille. It has features like a Blind Spot Monitor (only available on the most expensive MDX 'Advance'), rain-sensing wipers, advanced keyless entry & start (n/a on base MDX) and all the expected power amenities. The two-tone leather interior is also very attractive and tasteful and the assembly quality inside and out are top notch. The CX-9 is also available in FWD or AWD, while the MDX is AWD only (thru 2013). If you don't need AWD, it's just added mechanical complexity, extra weight and increased fuel consumption. If you need AWD, it's great, but I didn't and FWD saved me money and continues to do so.
I encourage you to at least take a look and drive in a CX-9. You could easily find a 2010-2012 CX-9 GT within your price range and with well under 100k miles. Oddly, Mazda made a few features optional that I think should have been standard. My 2012 CX-9 GT FWD has the Moonroof/Bose/SIRIUS package and Power Liftgate package. The only option I skipped was the Navigation system, but most GTs seem to have it.
Either way, I don't think you'll regret your decision....they're both excellent vehicles!