With the Nikon consumer DSLR now completely refurbished, a lot of questions are coming to me about "which one should I buy?" So I thought a few general comments would be in order.
First, the new generation definitely upped the game. While you'd expect that more pixels would cause image quality issues, especially at high ISO values, that's not been the case. If anything, the opposite is true. Sensor development is much like processor development: each generation makes a stride forward in ability. Sometimes those strides are small, sometimes they're bigger. This generation is one of those bigger forward pushes. Let me put it this way: once we get to ISO 1600 I'd rather use a D3100 than any of the 6 or 10mp cameras Nikon produced, and I'd rather use the D5100 or D7000 than any of the 6, 10, or 12mp DX DSLRs Nikon produced.
Even at base ISO there are some modest gains to be found (dynamic range is slightly improved), though from a practical standpoint these don't show up unless you're meticulous about image capture and processing.
Thus, right up front we can say this: if you're working mostly at base ISO it will only be the feature sets that really interest you if you've already got a 12mp Nikon DX DSLR. If you're working at high ISOs a lot, the current generation is a clear step up from where we've been in the DX line, and so you have to evaluate both on image quality and feature set.
So what about those feature sets? All three cameras went a bit upwards from where their predecessors were, with the D7000 taking the biggest step upwards and the D3100 taking the smallest. Curiously, all three cameras have a slightly different personality, and different strengths and weaknesses:
- D3100. Strengths: price, size, weight, simplicity (if you avoid the novice features, e.g. Guide), ability to mount pre-AI lenses (but not meter with them), best entry-level body Nikon's made in the digital age. Weaknesses: autofocus system (really set to Focus Priority and can't use non AF-S lenses), limited customization, only available with lens bundle.
- D5100. Strengths: best and most flexible video of the bunch (except no full manual control), positionable LCD, size (smaller than D5000 it replaces), upscale features (GPS, intervals, multiple exposure). Weaknesses: autofocus only with AF-S lenses, button scatter (controls not well placed/organized), no remote CLS control.
- D7000. Strengths: excellent build quality, excellent AF system, supports AI lenses, remote CLS control from internal flash, twin card slots, larger battery, optional grip, upscale features (GPS, intervals, multiple exposure, 100% viewfinder, more). Weaknesses: buffer depth in bursts, pricey for a consumer body, 1080P only at 24 fps.
Nikon makes it a bit tricky for someone to choose between the three. The D3100 is a bit like an entry model car: there are features that just can't be had, but the price is right. The D7000, on the other hand, is a bit overloaded with features, trying to get you to overbuy. Still, if you have older Nikon lenses (post AI but pre AF-S), the D7000 is the only model that fully supports those lenses, so it's generally the camera of choice for the upgrading Nikon user. Meanwhile, the best video capabilities were left to the D5100, and that positionable LCD just adds to that: if you're into video with a DSLR, the D5100 is your best choice in Nikon's consumer lineup.
If you've already got a previous Nikon consumer DSLR the question is whether this is the time to upgrade. Simple answer: if that older model is 6mp or 10mp, yes, but make sure you have good lenses. If you have a 12mp consumer DSLR, the gains aren't large enough for most people to consider moving up. As I've written before: update every other generation.
If you've already got a previous Nikon prosumer/pro DSLR the update question is easier to answer: if you really truly want to go from a pro-type body to a consumer one and you can tolerate a smallish buffer, then get a D7000. Otherwise wait. The prosumer/pro bodies are next in line for updating.