You can nominate guardians in your will but this is only an expression of wish and is not legally binding. Only the courts can make a legally binding appointment.
Generally both parents should make wills and agree who the guardians should be so the expression of wish is the same in both wills.
If you and your husband each make a will appointing different guardians then if you die first your children will live with their father and on his death the guardians appointed in his will would take precedence. However, your family could challenge the appointment through the courts and your children's own wishes would be taken into account providing they are old enough to express an opinion ( usually over the age of 7).
Guardianship appointments should always be discussed with the intended guardians first and the following matters should be considered:
Do they have enough space to house the children? Are they expecting to move into your family home?
Would the children need to move schools?
Would they have the funds to raise the children - food, clothes, uni fees etc or would you make some kind of provision in the will to cover the guardians' costs?
Who would the guardians appoint in their stead if they were to die ?
Lots of other considerations but these are a starting point.