As someone who's been steadily sizing up in width over the last 4 years, I actually think it's wheelbase. I've been on 8.5 with 14.5 wheelbase for the last 2 years, and felt like I had a hard time scooping some shuvit and 360 shuv variations. I got a xmas complete setup recently that's 8.25 with a 14.25 wheelbase.
The WB effects the way the board turns (or feels when you land on it), and for me, where I need to set my feet up for flip tricks. It's also noticeable on scoop-based tricks. I've gotten used to it for the most part, but thing if you're only varying the width within 1/4 of an inch, wheelbase is way more important.