Gotta love finding a well stocked fly shop!
As for the question at hand...I like some synthetics and don't mind them exclusively or mixed with traditional materials in a fly. That's part of the beauty of it, you can tie what YOU want in a fly. Synthetics won't absorb much if any water so that can be ideal in some conditions. Others perhaps not. I do like article fox though. I find it easier to work with than craft fur and I like how it swims.
There are no substitutes for good quality feathers though, at least none that I have found.
If you are tying a traditional fly and really going after the traditional way of doing it, the choice may be obvious. However sometimes you will have to substitute if the natural material is not readily available or illegal to possess.
Bottom line, tie what you like, enjoy it and more than anything, have fun fishing it.