How much can your payment move?
If a payment increase would strain your budget, a fixed rate buys certainty. If you have room, variable has historically cost less over time, though not always.
Might you break the mortgage?
Variable mortgages usually carry a three-month interest penalty. Fixed penalties can be much larger. If a move or sale is likely, this matters.
Consider a shorter fixed term
A two or three year fixed term is a middle path: stable payments now, and a fresh look at the market sooner.