I am running a regression model to find the impact of Federal Funds Rate(FFR) on Stock market.
I have monthly data over the period 2000-2014 and cross countries (Europe and America).

Why papers are always using stock returns and not log stock prices in Panel Data? is any econometric issue to believe that stock returns are more robust that log prices?
If I run:
Log(stock Prices) as Dependent not (Returns) is this an econometric issue ?