The difference is how you're going to set the expression itself.
For a 6 side dice you would use :
Choose(1,2,3,4,5,6)
or
int(random(1,7))
Choose returns only one of the parameters inputed, whereas random would return any number in the range, so adding int() and making sure the range is between 1 and 7 (random won't never return/reach the last bound of the range).
Also choose() can accept strings as parameters whereas random() only deals with numbers (float).