D3S vrs D5100
I always prefer the feel of my D3 and D700 over the 5100 (kind of like prefering the feel of a BMW over a Chevy) - yet the D5100 is often the one I grab. It's small and light, so easy to carry (conceal), and and with the low cost, I'm not overly concerned about it being damaged or stolen - so I'm more likely to have it with me.
But the main reason is with the increased pixel density over D3/700 sensor, I get significantly more detail resolution compared to the D3/700 sensor. The D3/700 sensor is clearly better at low light, but not as much as I would have initially thought. If you're typically shooting low light, the answer is more clear, if you're shooting high volume, same thing - but if the light it pretty good most of the time and you're not shooting hundreds of shots at a time, the D5100 seems to be a pretty nice alternative.
At least in my opinion, you've made the right decision to focus on glass, which is where I thing the biggest gains are to be had. Ironic though, DX cameras can often do better with poor glass than FF cameras as they only use the sweet center of a lens, typically the best part of even lesser lens, which often fall apart around the edges/corners. You've got some good glass, which the DX can take advantage of.
There is a magic to bigger sensors though, and I'm guessing the answer in 35mm'ish is the newer D800/e which gives you roughly the same pixel density as the D5100 and the benefits of a larger sensor as well, so if you were starting from scratch, I'd get that one - but if I were you now, I'd stay with the D5100, sell the D3s to someone who needs its particular strengths (superb low light, built like a tank, runs forever, silky operation) use your D5100 with your good glass and take wonderful photos until you decide to take your camera and glass to the next level.
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