Servos - Coreless or Digital?
Most digital servos are coreless... only exception that I know of is Hitec's 5600 series servos whiich are 3 pole motors.
Digital or Analog... Coreless or Not... The determing factor is your application and ultimately your budget.
Depending on your application and budget a good analog coreless servo im most cases will do just fine but if your ganging servos you'd be better off to use digitals.
What are you planning to use them in and maybe we can give you a better recommendation based on what you are using them in.