once youll start using the D610, you wont want to use the D300 as much. I had the D300 and got a D3, then saw the IQ, and I was bummed with the D300. pictures look flat compared to FF. huge dynamic range, grain quality is nicer. high iso, no comparison. DX cant do more than 2000 at most and its suffering from heavy smearing and chroma noise.
then I got the D600 and when loading images in ACDC pro for the first time, I was confused to what I was seeing. it looked like the resolving power was better. and yes, it was. so much more. thats why the tamron is superior than the 24-85 because it will take advantage of the sensor much better.
I use the D600 for family formals because theres more detail there. now the D3 images look soft to my eyes compared to the D600. its a never ending "I want more"race. file sizes are heavy on my pc though. theres a very noticeable delay when editing D600 files. even annoying at times. I use jpeg fine L. problem is the ergonomics and speed (Af, shutter lag, FPS) and no lag with the D3 keeps me coming back to it so im in between them. its just a superior machine technically wise. IQ wise the D600 is clearly the winner.
just watch the chroma noise at iso 3200 and above, ull have to use the high iso NR. I use it at normal and that softens the image just a smidge but its the best balance. I shoot at iso 4-5000 on the dance floor
let me know how your screen is. mine has a greenish cast to it. oh, also, dont forget to sync date and time between both cameras so when you load files in the pc they go according to the event timeline. cheers!
also, I bought this grip
Vertical Battery Grip MB D14 2 x Decoded En EL15 for Nikon D600 DSLR Camera | eBay
includes 2 batteries as well. very nice for the price. oh,btw, I bought 2 just in case it ever fails and I will need it for weddings, I have a spare and at $30 for the grip itself, theres nothing to talk about,=. nikons costs $260
heres a video of it I uploaded