The big difference is how the decreases lean - left or right. That's why you do them differently.
There are many types of decreases, though, and you might want to try one or two on a sample square to see how they look to you.
is a good site to visit for ideas. Down at the bottom of the page are several links about decreases.
The thing is that you want the decreases on both sides to match, so your sweater will look symmetrical.
Hope this helps!