The guy who sits besides at work me does these calculations all day long and I asked him to run your scenario quickly. He told me that if you take variable @ 1.85% now and assume that every 6 months prime will go up by 0.5% up to a max. of 6% and each year you make $10K as annual prepayment and compared it to 3.7% fixed for 5 years with $10K prepayment. For both the terms you were making $500 payment/bi-weekly. You would will save approx. $1500 if you go with fixed rate over variable. I take no responsibility for these numbers
IMHO, you are probably better off getting a fixed rate and have the peace of mind but others may have different opinion...
http://www.vertex42.com/
He uses the home mortgage calculator sheet on this website...hope this helps!