The answer I used to give people to that question was if you can afford to take the risks of the ups and downs of a variable rate you will probably (note probably, kind of crystal ball gazing!) do better over the term of the loan.
However if a rate rise particularly in the early years could be a difficulty for you then go for fixed, be aware of potential decreases in income in coming years, e.g. childcare etc.
Lot to be said for the split option, kind of the best of both worlds if the variable rate is reasonable. You will gain from any decreases in variable on half but be protected from increases on the other half. Gives you the option to increase payments on the variable bit if you have the money or pay off lump sums should you be so lucky. Do you have to split 50/50?
There is no one size fits all, all depends on your own circumstances and your own attitude to risk.