Actually, if you want it to be nicer (sometimes you can get results like 3.03030303:1), you can change the formula for E1 to:
E1 = IF(C1>D1,LEFT(C1,4)&":1","1:"&LEFT(D1,4))
This will effectively round fractions to two decimal places. You can change this, of course, by using a number other than 4.