There is no prerequisite for "going digital". As with everything you utilize for your art, it's a tool. You get a choice in what tools you work with.
One advantage of working digitally at least (since you mentioned the lack of a scanner) is that the end-result is always "post-ready". I also have a crappy smart-phone, and prefer to scan for posts. But nothing beats the crispness of digitally-originated images……PLUS: you can work in any size or resolution that your hard-drive has the space to handle.
Basically: everybody above has hit all the other pertinent points………but one more thing:
You "acquired" the Cintiq from your friend. Since NOT having a scanner is an issue, I'm wondering……did you get the Cintiq for free, or at a substantially-reduced price? I only ask, 'cause, the LAST time I checked (which admittedly was several years ago) you could pick-up a fairly decent scanner for under $300 bucks (probably a lot less now).
If you have the means, I'd surely get one, if for no other reason than that we'd have the opportunity to see your traditional work in good resolution as well, and be able to better critique it, to help you progress. I'm not personally aware of ANYONE who works ONLY in one or the other medium (but that's just me……I don't know everything)
BTW: That Batman piece shows some pretty decent chops, for the stage you're at. Is that one of the ones you copied? In any case, keep it up, keep observing, and drawing what you notice.
Go digital now.
Don't listen to the superstitious types. Digital is the modern production pipeline, so the sooner you get your bearings with it, the better off you'll be.
Additionally, you have the advantage of not needing to scrap or completely erase wonky drawings (read: actual wasted productivity time), when you can make corrections by manipulating the areas that need adjustment.
Seriously, it is 2017 - digital will make your life easier, and your ability to work with clients that much easier. Pick up a copy of Clip Paint / Manga Studio the next time it goes on sale and go to town.
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Digital is just another tool just as a crayon or brush are tools. Neither helps you draw better.
You have to practice everyday and learn anatomy, light and shade, perspective and a host of other things.
It will help you to take some life drawing classes.
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Depends on what you want to do:
Produce work for a production pipeline or make art?
You want to sell original pieces to customers or work for hire on comic books, illustrations and so on?
Digital will make you faster, which is key for competing in the illustration market and keeping your costs in control.
Like the others said, if you don't know where to draw the line, you can't do it with a pen or wacom.
I want to do more traditional, picked up the pencil for my last match here again in ages and was loving it! The feel, the smell of paper, all of it. But with my busy schedule these days, drawing sessions that last for several hours are scarce. So with some practice and some dirty tricks I can do half hour sketches on my phone that would take three or four hours when doing taditional.
Going digital kept me from quitting drawing altogether these last two years.
Your life can change drastically as well, having some chops on your cintiq can save your ass down the line