It's always hard to tell, especially if you learn them at very different times. I learnt vertical box a long time ago (gosh, like 7 years ago or something now, that makes me feel surprisingly old!), and presumably CIB sometime after that Mura video (since that's where I learnt it from), so I may have found it easier through being a better juggler.
Plus I'm better at confusing tricks over just hard tricks.
Plus I think vertical box is hard for longer runs (which I worked on lots), due to it being surprisingly tiring, unless you are very efficient, so that may have been a lot of the difficulty.
by Brook Roberts, in response to this post 2015-09-11 16:38:33