Now that I think about it, the biggest reason I continue using fixed-width is that my preferred 2-space indent looks narrower and so indentation becomes less salient. Perhaps I should just increase my indent. 2 spaces is mandated at work, though, so there's that..
Try using indent guidelines if they are available on your IDE (built in or as a plug in). I work with two space indent, minified curly braces, proportional font, and indent guidelines...it is quite nice and feels "modern."
