In many games I race to Economics after Liberalism to get the free GM. This often means I have access to Free Market long before the AI.
Since both civics affect trade route income, I usually base my decision on what the AI is doing. What I usually do it check the foreign advisor's "Info" and tech screens. If I have a lot of foreign trade route income, and the AI civs are running Decentralization, and most of them are a ways off from obtaining Banking, I'll change to FM. But if most of the AIs are already running Mercantilism (which is often the case), it makes no sense to run FM when there's little or no foreign trade route income available, so Mercantilism it is.
Even after the AI has Economics, it takes a long time for them to change to FM. Even if you bribe them or use a Spy to get them to change, they often don't stick with it.
Sidebar: I've started playing Marathon speed lately, and in those games I've noticed the AI's often skipping Mercantilism altogether, going pretty much straight from Decentralization to FM--unlike Epic speed, where I find the AIs love Mercantilism, adopt it early, and run it for centuries. I'm not sure why the game speed difference makes that change.