Which camera should I buy?
Interestingly, at this point in the market, there's not a huge difference in capability or intent between the mirrorless models and the conventional DSLRs. As with less-expensive DSLRs and mirrorless cameras, there's no difference in image quality, but at this higher level, there's also no real difference in terms of controls or usability. Those differences that remain - which mostly relate to continuous focus ability - are decreasing all the time.
Best enthusiast-level camera - Olympus OM-D E-M1
It's a close-run thing - it's hard to imagine anyone being unhappy with any of these cameras - but we think the camera that stands out most in this class is the Olympus OM-D E-M1. It's not the highest scoring camera in this class, because it isn't the very best for image quality (which our scoring system is heavily weighted towards) and because our scoring doesn't factor-in the E-M1's portability.
The E-M1 offers the same handling and build qualities of its rivals, but with a distinct size advantage and with a lens range solely designed for its format (rather than lens line-ups padded-out with full frame lenses that aren't optimal choices for APS-C shooting). And, most importantly, image quality is genuinely competitive - though you'll need fast lenses to make the best of it. Fujifilm's X-Pro1 is also worth serious consideration, but the arrival of the X-E2 below it highlights some of the benefits we can expect, if/when it's replaced - slightly undermining its considerable appeal.
Best enthusiast-level DSLR - Nikon D7100
There are, of course, some reasons why you might want to choose a DSLR over a mirrorless camera - continuous focus tracking being the main one. Beyond this, the age of the remaining DSLR systems means there are some specialist lenses (tilt-shift, unusual primes or high-end sports telephotos), that aren't available for mirrorless cameras yet. If you need any of these things, or are simply attached to the idea of using a DSLR, then this is the way to go.
It's extremely hard to choose between the Nikon and the Pentax, but the sophistication of the Nikon's AF system just puts it ahead, for us. The margins between products are small in this category, so if you already own any good lenses for the Nikon, Canon or Pentax system, we don't think the differences make it worth changing. But if you're new to this category, definitely want a DSLR and aren't already committed to a particular system, we'd go for the D7100. For everyone else, we'd suggest taking a serious look at the mirrorless options.