For most people, most of the time, the issue is qualifying for the mortgage, not the price differential between the old house and the new house.
The problems is that periods when prices go down are often times when interest rates go up, like now. It tends to be a wash, since you wind up with the same monthly payment.
You have a particular situation which may bias your decision towards "buy now", which is that you have an ARM. Since your mortgage rate is going to adjust in two years, the fact that we are in a period of rising interest rates means that you are going to either have move up or refinance.