I'm really tired of these tabs vs. spaces discussions. There are good reasons for either approach, and I don't see why one approach would be _objectively_ better than the other.
People who prefer the pro/con combination of tabs over the pro/con combination of spaces, prefer tabs. People who prefer the pro/con combination of spaces over the pro/con combination of tabs, prefer spaces. It's subjective. My recommendation is to follow a project's existing approach or, if you start a new project, the "community standard" approach (two spaces per indent for Nim). I think there are really more important things to discuss than tabs vs. spaces. (Yes, it's ironic that I still took the time to write this reply. ;-) )