Up to now, my analysis was incomplete - we need to take the sample rate into consideration as well:
SDS800: 2 x ADC, 1 x 100 Mpts, 1 x 2 GSa/s
SDS2000X HD: 2 x ADC, 2 x 200 Mpts, 2 x 2 GSa/s
SDS3000X HD: 2 x ADC, 1 x 400 Mpts, 2 x 4 GSa/s
SDS6000: 4 x ADC, 1 x 500 Mpts, 4 x 5 GSa/s
So there might actally be differences between SDS800, 3000 and 6000.
I honestly don't know what drives the decision to organize a single shared memory vs. corresponding chunks of memory per ADC. Single shared memory quite obviously has the advantage of longer record lengths in single channel operation.
It might be that the architecture of the SDS3000X HD would be the same as the SDS2000X HD, hence only 200 Mpts max. per ADC, and we only got a special solution that allows a single record utilizing the full memory. If, on the other hand, the history can't get close to even 200 Mpts, I don't have any explanation for that.