I would like to throw in the idea of not trading entire systems but going from a semi pro body down to a smaller one.
I used to shoot with a D300 and it broke me. Now I shoot with various M4/3 bodies with the nice glass, the fujifilm, X100/x20, a entry level sony A37 with my old 1990s minolta glass + my 90mm macro lens (I cannot say enough how hard focus peaking rules) and a D5200 with primes.
I would really
really urge people to look at what lenses they would end up buying before the bodies and compare it to what they have. I ended up with the two DSLR bodies because the
second hand price of them has absolutely tanked since people, and by people I mean people like my mother who only take holiday pictures, decided they wanted M4/3 instead.
I paid £150 for a A37 with 100 shutter actuations and that's just madness - the D5200 was ex-display and £300 and that's full blown they are coming to take me away haha insanity - this is a camera with the 7000s focusing system, the D7100's iso stuff and a newly developed sensor. Stick a 35mm 1.8 on it and you have a astonishing camera you can't fail with for about £420.
Both of these bodies are sort of half way between the smaller body cameras and D300/D200/D7X00 in weight, stick a 35mm/50mm prime on them and they actually weigh about the same as the oly Em-5 with the 25mm f/1.4 on it.
But anyway, I'm rambling now, as I said it's all about the glass, and for the love of god don't buy into the nikon or canon smaller sensor/body system, it's not that they are bad, it's that the competition is light years ahead of them in all ways.