the different generation algorithms are largely a resource/time trade off
if you have lots of extra RAM in your design, choose one of the ROM based NCOs
if you have lots of logic leftover use a parallel CORDIC
if you've got a high clock rate but low sample rate use a serial CORDIC
if you've got some multipliers left over use a multiplier based
a common specification for NCOs is spurious free dynamic range SFDR. you can get an idea of what you can achieve in the MegaWizard when you vary the parameters
