If you use AutoCAD to make models or drawings, you still need Aspire or another CAM program to create your g-code. If you're good with AutoCAD, it has far more drawing tools than Aspire, and should be faster. But it can take a long time to learn well, as it has so many features.
Many people use Aspire only, and find they can do everything they need with just Aspire.
And Aspire is probably easier to learn.
I'd say if you don't know how to use either, stick with Aspire, unless you find that it can't do something you need.