OK, I've crunched some numbers. For both scenarios I've assumed that;
- the loan is for £133,000 over 28 years on a repayment basis;
- after the initial discounted rate ends, it reverts to a standard rate of 4.50%;
- the monthly repayments are calculated to be the same throughout the term.
Assumption 3 may not necessarily be sound. I don't know how banks and building societies actually calculate monthly repayments. I've done it as if they knew in advance that you'd be switching to 4.50% at the end of the introductory period, and wanted to ensure that your payments didn't change. In practice, if they did it like that, then they'd have to recalculate your payments if the standard rate turned out to be something other than 4.50% by the time you were switched onto it; but then they'd have to recalculate your repayments every time the standard variable rate changed anyway. So that sounds pretty sensible to me, but remember I said I'm good at maths, not necessarily good at banking.
Scenario 1.
£133,000 over 28 years. First 2 years at 2.70% and then switching to 4.50%.
Monthly repayment = £668, total interest paid = £91,337, total repaid = £224,337.
After 5 years you have paid £40,060 and the outstanding balance is £115,672.
Scenario 2.
£133,000 over 28 years. First 5 years at 3.50% and then switching to 4.50%.
Monthly repayment = £662, total interest paid = £89,424, total repaid = £222,424.
After 5 years you have paid £39,719 and the outstanding balance is £114,685.
There's really not very much in it at all.
But the calculations are moderately sensitive to the interest rate which applies at the end of the fixed rate deal. For example, if it were 6.00% then it would tilt the odds very much in favour of the 5-year fix, because the longer you could delay having to pay 6.00% the better. In this scenario, 5-year fix would save you £30 per month, and £10,000 over the duration of the loan.
I've saved the spreadsheet so I can re-run it with any alternative assumptions you'd care to test.