in is peferable bcoz
by using in we can write set of values at time n check it 
where as when we use or we need to mention for each n every 
value
like ...
select emp_id from employee where emp_id in(1,2,3,4..)
but in case of or...
select emp_id from employee where emp_id=1 or emp_id=2 or 
emp_id=3 or emp_id=4...