Single loop containing multiple if-statements or multiple if-statements with loops inside?

Is it recommended to create multiple if statements and nest loops inside of them or create one loop and nest multiple if statements inside of that? I am wondering in terms of memory, time, and other factors I may have overlooked.

Here is some pseudocode:

if a: for i in range(500): do x if b: for i in range(500): do y

versus

for i in range(500): if y: do x if z: do p

In other words, when is the best time to do the if-statement. Inside a few nested loops or before any of the loops?