On a mobile device especially, spaces are problematic because it takes so many presses of the backspace key to delete heavily-indented code. That's why I started using tabs. (This would be less of an issue if, when pressing backspace at the beginning of an indented line, it deleted to the beginning of the line, or at least to the next-highest indentation level.)
Tab inserting spaces would be nice to have but I can live without it. I would prioritise line numbers over it It would be even nicer to have the number of spaces configurable but a hardcoded 4 would suffice.