Trying to merge 2 vector layers; one is of type "LineStingZ", the other is just "LineString". When I run the merge algorithm I get the message saying "Error executing algorithm Merge vector layers All layers must have same geometry type!"; but when I try to do a save-as on the LineString layer and check the 'Include z-dimension' I get a new "LineStringZM" type and they still don't match. Is there another way to specify the layer type?
You should be able to use the MMQGIS plugin to convert your LineStringZ layer to LineString. From the MMQGIS menu go to Modify > Convert Geometry Type. Select your LineStringZ layer as the source layer and new geometry type as lines.
EDIT: As pointed out by @Alexandre Neto, in QGIS 2.18, Convert geometry type is available from the processing toolbox under QGIS geoalgorithms > Vector geometry tools.