Yes you can draw in both. Illustrator is a vector drawing program, so use it as your primary tool for creating vector art (paths and anchor points, unlimited scaling).
Fresco is mainly for pixel-based art, but it does have a vector drawing tool, which makes Fresco a hybrid that can create both types of art.
In Fresco there are brushes that paint in pixels and can create realistic textures of paint and charcoal and other traditional media - that's not something you can do in Illustrator. You can look on Behance and search for art created in each app to see examples of what it looks like.
Although Illustrator might seem like a fine art application used for drawing stuff freely, it is used for almost anything from CAD related stuff to (multiple) page layout, logo design, fashion (from sketch to finish), packaging design, and of course illustration/fine art.
It is suitable for doing prepress and other production stuff.
Fresco can simulate traditional (analog) media and is used mainly for illustration/fine art. You cannot do any construction stuff or anything of the other things listed above. And while it might be possible to get the artwork directly into print from it, I wouldn't recommend it.
But you can create animation and it's really cool for sketching.