One question I always have is comparing the cost difference between variable and fixed rate mortgages?
A fixed interest rate will not change during the fixed period. During the fixed period the borrower knows their repayments will remain unchanged.
A fixed rate loan is also advantageous if variable interest rates rise. When variable interest rates rise a borrower with a fixed interest rate is relatively better off because their rate will remain unchanged.
I have found a unbelievable online calculator that is based in Australia but it works great.
It is available here, http://www.yourmortgage.com.au/calculators/fixed_variable/
You can even enter how you expect the variable interest rate to change over the period of the mortgage so it makes it great to do what if analysis!