The 13.5 fl. oz bottle is better, because it is cheaper per ounce.
The 16 fl. oz bottle is $6.19, so it is $6.19/16oz = $0.38/oz
The 13.5fl. oz bottle is $3.99, so it is $3.99/13.5oz = $0.29/oz
0.29 is less than 0.38, thus the 13.5 oz is cheaper and the better buy.
