Assuming the storage size is important: 
I have a long list of digits (0-9) that I want to write to a file. From a storage standpoint, would it be more efficient to use ASCII or UTF-8 as an encoding? 
Is it possible to create a smaller file using something else?
There is absolutely no difference in this case; UTF-8 is identical to ASCII in this character range.
If storage is an important consideration, maybe look into compression. A simple Huffman compression will use something like 3 bits per byte for this kind of data. If there are periodicity patterns, a modern compression algorithm can take it even further.
ASCII vs UTF-8?