I used to prefer tabs (for the exact reason, tabs indicate an indent, spaces indicate a space) except for one edge case that has me using spaces now. If you're anal about making sure chained method calls line up on their own lines, spaces guarantee that across the board, but tabs would mess up the indent.

  var foo = SomeTerriblyLongHorrificClass->withUnreasonablyLargeChildren()
                                         ->dueToTheOddNumberOfCharachters()
                                         ->sometimesItsNiceToUseSpaces()
                                         ->here()