I've done multiuser editing with QGIS and Esri (note: QGIS is no slower than ArcGis at capture/update), however we have and use SDE; I have found the problem is not QGIS but ArcGis...
ArcGis on SDE for PostgreSQL using PostGIS geometry types allow for the data to be 'shared' around between users which works great in a read-only sense but there are significant differences between the way that ArcGis and QGIS write to PostGIS databases...
QGIS writes directly to the database so as soon as the user saves edits the edits are visible to other users in QGIS or ArcGis as the change is made to the base table.
ArcGis uses versioning to edit PostGIS data where the edits are made against a version and stored in a delta table, when the user saves the edits are visible to other ArcGis users - as ArcGis is aware of the delta tables but QGIS users are not aware of the delta tables so the edits made in ArcGis are only visible to QGIS users after the database has been compressed back to the default state.
I would recommend that you ONLY EDIT THE POSTGIS DATA WITH QGIS and use ArcGis just to view the data; it's also far more convenient to build the database in catalog and export using toolbox tools. Don't panic, you still can use your geoprocessing tools in ArcGis, build topology/network make cartographic maps etc.. all the fancy stuff - just don't version the database and edit in ArcGis.
P.S. I back up daily on a scheduled task using pg_dump which can be restored using pg_restore - try your backup/restore process on a test dataset as not every backup method will produce a viable PostGIS database when restored (the tables will be there but the triggers may not work) and that's something you don't want to find out the hard way.