Practically speaking, the 80486's main advantage over the 80386 is just that the 486 is faster. Even if you compare the two chips running at the same clock speed, the 486 will generally finish its calculations sooner, sometimes much sooner, than the 386.
There are at least three versions of the 80486 that you may run across. The standard model, called the DX, includes a built-in math coprocessor, whereas the sx doesn't. pcs based on the sx are a little cheaper, but if you decide to add a math coprocessor later (it's easy to do), you'll spend more than if you bought a DX machine to start with. And there's the DX2.This model does its internal computations twice as fast as a standard DX running at the same clock speed, slowing back down when it's time to move data to or from memory. The DX2is decidedly faster than a standard 486DX,and not much more expensive.