As far as I know, there is no difference between concurrent and semi-concurrent filing nowadays. If you are interested in getting the benefits of I-485 (e.g., AP or EAD, or even to change job for cases except self-petition under AC21 memo), then filing the I-485 sooner is better. VSC has frequently adjudicated concurrent filing cases simultaneously (e.g., I-140 and I-485 cases for Nitin, jollyfella, 2006gc, etc. were approved on the same day). But VSC no longer adjudicate new employment-based petitions. The only drawback for concurrent filing and semi-concurrent filing is that I-485 is automatically denied if the underlying I-140 is denied.