Yes I've heard the theoretical arguments that an internal clock ought to be better. And everything else being equal (phase noise of oscillator, level of noise in clock circuit, quality of power supply, shielding etc) I'm sure an internal clock would be as it cuts out extra connections. Very low phase noise oscillators and associated circuitry aren't cheap and nor would the so would seriously push up the price of dacs.
Anyway like much of this stuff, the proof is in the pudding, it's the best $450 I've spent in hifi.
The $1.5k C18 clock is specced at <-110db/1hz, same as the $450 OCK1, with the $750 OCK2 5db better than both at -115db. Normally to get that level of performance you have to spend well over $2k... so the OCK2 looks like a game changer IF its performance matches its claimed specs.