When should I choose 64×100G instead of a switch with 400G ports?
When the optic plan is 100G only. RG-S6520-64CQ lists 64 × 100GE. RG-S6580-48CQ8QC lists 8 × 400GE as well as 48 × 100GE.
Is the higher capacity always the better buy?
No. 16.0 Tbps on the 400G PID does not help a design that will never use 400G optics.
Where is the full comparison?
What if I only need about 32 × 100G?
Start with CE8850-32CQ-EI instead of either 64-port class switch.