There are two basic differences between those two models, the first is that the 3660 has been beefed up a little in the leg area of the stand and the second is in the way the saw is packaged. The 3650 used to come in two boxes and the 3660 only comes in one. Other than those two differences they are the same saw. That's a great price for a new in box 3650, just make sure you get both boxes as box 2 of 2 contains the fence and rail system.
Life is like a roll of toilet paper.
The closer it gets to the end, the faster it goes.