The question you need to answer is a simple one with a no-brainer answer - What direction are interest rates going to go in the next 10-20 years?
BTW, the answer to the above question should also point you completely away from an adjustable rate mortgage.
At current lowest ever rates, you can win three ways. First, get the lower rate and pay less interest over time.
Second, you REALLY need to think about looking at a 15 year mortgage. The payments will be a bit higher, but the contribution to principal is MUCH higher and much faster. Upon a sale, the accelerated equity accrual can yield a very nice result for available funds to purchase the next house.
Third and best. The amount you end up paying on a 15 year mortgage is incredibly less than on a 30 year mortgage.
Get yourself an amortization calculator and play with the permutations. Once you see the difference between a 15 year and 30 year mortgage, you will be stunned.