Your question is actually a common one: What to do with the passion project you’re really interested in writing, but worry you don’t have the skill to accomplish. I was in a similar spot when I was younger, only I was ignorant of the “lacking skill” part of the problem. Needless to say, I just launched into the project. It wasn’t as big a disaster as you might think. Oh, the book was pretty bad, but the learning process (and my own excitement for it) was of great significance for me as a writer. However, I did stop the book in the middle as I got lost, weighed down by the plots and characters, and didn’t start another novel for several years.
At a different time in my career, I tried to write The Way of Kings. I was a much better writer then (I’d already written Elantris) but KINGS was still too big a project for me. I finished it, but it was something of a mess. I’m glad I wrote it, though, because by then I was skilled enough that a failed book taught me what I needed to know in order to write it the correct way.
Pondering this question, I think (like so many questions as a writer) that there might not be a ‘right’ answer. What I can do is weigh some of the advantages and disadvantages of each situation.
The biggest problem with writing your passion project is the possibility that your skill will leave you disappointed in how it’s progressing. While it’s possible it will just go beautifully, for most of us, it can be very frustrating when the vision of a book we have in our mind doesn’t match the thing coming out on the page. I’ve known people who get very discouraged by this and quit writing.
The advantage is that you should generally err on writing the thing you’re most passionate and excited about. Writing something you don’t want to write can be a disaster, as once you get into the thick of the story, it’s going to be difficult to continue. (This happens even on books you love, and you can imagine it’s worse on a book you feel is ‘just for practice.’)
It comes down to your personality, I’d say. Will seeing your perfect story get somewhat mangled (as will probably happen) be discouraging to you? Or will you enjoy the passion of it so much that you’ll accept its flaws, and your own limitations, with the understanding that you can fix them in revisions once you’re a better writer? Conversely, will writing on something you’re not as excited about hold you back–or, instead, are you someone who tends to get into whatever you’re working on? It’s entirely possible that the smaller, more manageable story will become increasingly exciting for you as you work on it.
Good luck to you! Just make sure you enjoy the journey, and focus on why each chapter–individually–is exciting and interesting to write.