Hi Jose,
The 6451 is 8 bits wide and is the 'standard' logic input probe that comes with the 308 analyzer. The 6406 is a 16 bit probe that only sends a trigger signal to the 308; for example you could use the 6406 to monitor the address bus on a small microprocessor and then use the 6451 to read the data bus when the 6406
recognizes a specific address or address range.
That said, what would you want for them?
Regards,
Waz