That seems to be the base model.
Not necessarily. That's just a generic car comparison website, and it sounds like they lump them all together. This isn't unusual; many sites consider the MDX versions as one "trim level".
BMW charges extra for many features, whereas Acura includes most of them as standard equipment, and offers a few optional packages of high-end features. Some of the things you'll pay extra for or are not available on the X5, that are standard on the MDX, include leather seats, heated seats, power moonroof, rear-view camera, perimeter lighting, and privacy glass. That's why a base MDX for $43K list compares more closely to an optioned $55-60K X5 rather than a base $48K X5. On top of the base model, there are three main optional packages from the factory for the MDX: the Technology package, the Advanced package (which includes everything in the Technology package plus additional features), and the Entertainment package (a rear entertainment system, including heated second-row seats, which can be added to the Technology package or the Advanced package).
Compare features and decide which, if any, you prefer, on both the MDX and the X5.
Also, if you're willing to buy a late-model used X5, you might also want to consider a late-model used MDX, for even greater savings.
Which car is best? Drive them and find out which one is best FOR YOU!