I'd keep one of the FX f/2.8 zooms lenses if I were to make this decision assuming you have the any of the 14-24, 24-70 or 70-200.
With so many people with quality cameras out there these days, a lot of quality pictures stems from being at the right place at the right time or from having work opportunities.
I don't see any attempt from manufacturers to try to have the DX sensors equal the D3/D700 sensor for at least 2 iterations after this current 24MP sensor. If there is one thing I'd envy with the D300 compared to the D5100, it would be the low-light autofocus. Maybe an SB-400 update with a small but much better autofocus assist lamp compared to one found on cameras would work for me.
