Sun 13-Oct-13 06:13 AM | edited Sun 13-Oct-13 06:21 AM by jamesvoortman
All three cameras have very similar controls, body style and structure of menus so you will feel right at home with the D800. Although they look and feel almost the same there are subtle improvements everywhere in the D800.
I loved my D300 and got some great pics with it. It went on lots of backpacking and climbing trips with me, survived heat, humidity, dust, snow and being bumped around a lot. I kept it as a spare when I got the D800 and have never used it since.
As Linwood says, the D800 is better in all respects, even in DX mode. It's DX pics are 15.5Mp and with much higher ISO capability than the 12Mp of D300. Keeping the D300 for the so-called "extra reach" of DX is a non issue. I see from your profile that you have only one DX lens and all the others are decent quality FX lenses so there is not a compelling reason to keep the DX option open
In your position I think I would opt to keep the D700 instead of the D300 because : - it has much better high ISO capability - it has better frame rate than both D300 and D800 - this could be an issue if you shoot sports.
Regarding the left focus issue ... I think it is over-hyped. I tested mine with a 105 macro lens fully opened and luckily did not detect any abnormality. I have not heard of too many others who had the problem or could not get it resolved....and Nikon has not yet seen fit to release a D810 in response!