Well that’s kinda the crux of this thread; the relative difficulty of 50’s vs 75’s. Coming from the perspective of someone in the water, 40 50’s would be a helluva lot easier than 30, or even 25, 75s at the same pace.
I should add, that’s assuming they’re both on :20 rest. If the 50’s were on :15 rest? That sounds equally as difficult as the 75’s on :20 rest.