Margins:

The margin is the amount of interest above the index rate that the lender charges, i.e. the true rate is made up of the index plus the margin. Given the same index, therefor, the more attractive loan is usually the one with the lower margin, though a higher margin loan may have less fees or a lower start rate. Margins between indices are more difficult to compare, and the best guess would be in looking at historic data for each index.