The differences are in the "Input Offset Voltage" and "PSRR" specifications.
MAX9698B
Input Offset Voltage +/-6mV (typ), +/-9mV (max)
Power Supply Rejection Ratio 50db (min), 65db (typ)
MAX9698
Input Offset Voltage +/-1mV (typ), +/-3mV (max)
Power Supply Rejection Ratio 70db (min), 85db (typ)
Unfortunately both devices are obsolete.