I guess the most obvious difference between the two is that gradients, while being easier to apply and edit, are limited to radial and linear gradations, while blends can better follow an object or path's contours as long as the points correspond correctly.
In addition to what Dave said there are differences in applicability. Try creating an Artbrush with a simple gradient to form something like a bent rod. Oh that is easy: Just create a rectangle with a linear gradient with a stop in the middle for the pale colour, no? No.
I want to thank you for your insight both of you have been very helpful.
You should also be aware that gradients created using spot colours will separate as shades of spot colours (i.e. maintaining spot channels).
On the other hand a blend between spot colours will result in the steps of the blend being converted to CMYK or RGB depending on the colour mode of the Illie file.