Mostly in history as described in another thread in this new forum. Another thing is like most denominations it's easy to join the UMC just by convincing the pastor and congregation that you're a Christians and saying vows the pastor gives you which basically say you will be loyal to God and the UMC. The Free Methodists have a year long Bible study requiring you to read the Bible from cover to cover. The Bible study emphasizes salvation so it serves the same requirement but takes much longer to join.
The UMC is the 3rd largest denomination in the US and the denomination found in the largest number of places in the US with congregations in a much larger number of counties spread out across America than any other denomination. Free Methodists are a small denomination and in the US they are primarly in the midwest. The city I live in, like most places in Texas, doesn't have a FM congregation and the vast majority of Texans have never heard of the Free Methodists. In the past couple decades the UMC hasn't emphasized missions like they used to but FM's have continued to do missions work so 2/3's of FM's live outside the US while most UM's live in the US.
One very important difference is the FM's for the most part are evangelical while the UMC during the last 50 years has been becoming less evangelical so although there many very evangelical UM's they represent the minority of the UMC. A related difference is that the Free Methodist Church is growing both in the US and around the world while the UMC is evangelical and growing in Africa, Asia and Latin America its members in the US often aren't evangelical so membership is stagnant inside the US. Also the UMC is divided between liberals (particularly in the western states and Illinois), conservatives (particularly in the south), and moderates (the largest segment of the UMC) while the Free Methodists have kept liberal or progressive theology out for the most part and are mostly conservatives with the rest being moderate.
There are slight differences in church government but they are very slight and I don't know enough on that subject to comment.