If you are both buying and selling and the market is similar; if the new home is $700K and the old home is $600K, both at 5% lower than last year. You'd be saving $35,000 on the new home and losing $30,000 on the old home, with a net difference of $5,000 savings if you swap the two homes.
Of course, you are going to incurr the expenses but you would have that no matter when you sell or buy. .
Another advantage to me is that since property value is going down, the property tax for you new home will be relative lower if you buy it if you wait.
So you are correct, if the market conditions in each town are comparable it makes no difference if the market is up or down ( keep in mind to budget between 5%-8% for all your fees, commission, tax stamps closing costs, moving fees ). So even if the home values are the same.. it's not REALLY an even swap.
Also consider that the market affects different towns differently... do you think Dorchester has fallen more or less than Hingham? It's not even close.. Dorchester has been slammed while Hingham is OK.
Quincy is where I live, and one of the town that I have constantly updated market information for, including hard to find info like all sold homes, list vs sell statistics and more. I work in Scituate as a buyer agent.. but do not catalog Scituate statistics.