Hi Roy,
It's a pure guess. If you take the a close mortgage you pay a bit more but you know exactly how much will you pay at the end (or PV). You simply pay a relatively small "insurance" fee.....
If you take the open one you "gamble".... you can save or loose all depends on the prime. My guess is that the prime will go soon up and in an unprecedented rate. I predect a shortage of cash in the markets (already started). That's my guess. I might be wrong. If you want to sleep well at night, take the 5%.
Best regards,