Do not use anything but a fixed-rate mortgage for the first mortgage
Let's repeat that. Do not use anything but a fixed-rate mortgage for the first mortgage. Forget about "adjustable," "option-arm," "interest-only," or whatever permutation the industry is calling such mortgages. For most people, they're dangerous at best; at worst, they're a way to get people who simply cannot afford to buy a home (or, at least, that expensive a home), to buy.
The exception? If you know your income is going to at least triple in the next 3-4 years, you can consider such mortgages. However, this exception comprises a tiny portion of the population and, even then, you should be very leery of such non-fixed rate mortgages. The bottom line; with interest rates as low as they are today, don't put... the ownership of your home at risk by choosing any form of adjustable rate, first mortgage.