Will I notice the difference between ADC Sampling Rate of 122.88 vs 245.76 Msps (6400 vs 6600)?

Simple answer is no.
The 6600 has two ADC, and the 6400 has one. Both radios share the same type of ADC and perform the same, but with two ADC the 6600 can do more things, feature wize because of it.

The ADC's in the 6600 each have a higher sampling rate 245.msps as does the 6500 and 6700. The ADC in the 6300 and 6400 do 122msps. The higher sampling rate will be more evident with the 65,66 and 6700 as it allows zooming the bandscope down to 1Hz resolution. That also allows for wider panadapter display, 14Mhz in the 65,66 and 6700 vs 7Mhz max bandwidth per panadapter in the 63 and 6400. So there is a visible difference. How useful that might be otherwise is up to your specific needs. James WD5GWY