If you can only get one of them, what about getting a E2 or E2e, and using it with the Z20 battery spacer. That way, you can run an E1 lamp, on 1-cell, but have a choice of reverting to the 2-cell configuration at will.
It is possible to run it with a home-made bypass, so that your "spare" cell is a fully charged 123 instead of a Z20 blank spacer.
The E2 is so small that you can still EDC it almost as easily as an E1.
lightlover