I would highly recommend building Jason's design, it is a proven engine and a very nice runner. I am 3/4 the way through mine with not much left to do.
The real bonus with Jason's is all the advice, methods and pictures he has produced for the making of the parts and the build. I learnt a lot on both the lathe and mill in following his how to make advice.
I looked at the Julius design but at my level of experience I think I would have struggled.
Ron