Having used both Adobe Illustrator and Anime Studio for many years and proficient with both apps, the time required in each to produce the same character is the same for both apps in my humble opinion. Of course if I do it in Illustrator I double my time REDOING it in Anime Studio.
Once you know what you are doing in both programs getting to the end result is a matter of drawing skills and manipulating points.
The process in both apps is different, and in some cases VERY different, but at the end of the day it's how the resulting creation is to be used. If you are invested in Anime Studio for animation (great choice by the way) then it makes so much more sense to use that tool for creating the characters originally since it will save time, and allow "testing" your creations to see how they animate in the program you are creating them in. In both cases it's probably going to start with some sort of "free hand sketch" scanned in or drawn with a stylus and traced in either application.
Bezier vector apps will produce fantastic final results, but without a way to "test" how it will animate the resulting creation will most likely have to be drastically altered to work in Anime Studio. Even if we had "perfect" import of bezier based vector files with points exactly where they were originally placed, those points will have to be moved and additional points added or removed, in order for the character to animate correctly. Often drawing a "still" image allows cheating the placement of points and their curvatures that would prevent good animation. When creating in a "still" based drawing app, you can't see how well your shape and point placement will work when animated. At least not until you get it into Anime Studio and try it out.
Often I draw or trace, say... an arm for the character. The arm has to bend at the elbow. If I simply drew the arm on paper, or in another drawing app, I am trying to make it "look good" and not thinking about where to place points so it can bend. I would also draw what the arm should look like when bent at the elbow, but still it's purely a visual reference.
Once in Anime Studio, I trace that arm and place points where I think it will work best to bend the arm smoothly and look good. I then add bones to test out the motion. I can tweak and adjust the point position, add some smart bones to smooth it out.
I could never have predicted where those points should go when drawing on paper or in Adobe Illustrator or even in Anime Studio, until I get to that point when I'm testing out the motion. There is just no way that you will ever draw something in another medium and import straight into Anime Studio and magically have a character ready for animation. There is always going to be tweaking and adjusting.
Some people can draw in Adobe Illustrator as quickly as drawing on paper. In that case it doesn't matter what program you use to sketch out the original character designs. Using other vector apps to do this is fine if you feel comfortable with it and it helps to get your ideas and designs created quickly. You can simply use those as tracing templates in Anime Studio.
The main thing to keep in mind is that in Anime Studio, point placement and shape creation is critical to animating a character well using bones. Points need to be placed in such a way as to allow those shapes to bend and move properly. Layer order, shape order, masking etc. All of these things need to be addressed in Anime Studio to determine the best way to set it up for animation. Your original character designs, whether drawn on paper and scanned, or drawn in Adobe Illustrator, should always be considered tracing templates to be redone in Anime Studio.
Just my 2 cents.
