I'd use P-input for better noise and such. Running as an error amplifier, the higher the gain the better. The higher the gain used, the closer the amp will set diode nodes to and therefore the more accurate and temp. independant the circuit will be. This being said, if you use 2nd order compensation, a simple 1-stage amp should suffice for getting below 10ppm/°C. Since you have the headroom though with respect to supply voltage, I'd suggest a 3 stage amp with 100dB of gain. I picked these numbers simply because they will yield awesome results and with a supply of 2-3.3V, you obviously aren't worried about power consumption.