The problem with the higher wattage laser diodes is the divergence (beam size when focused) is larger and does not give as much detail when engraving photos. However they do cut materials easier/faster because of increased burning power. Like I said, we have not tried the 3.8W, but I do know the divergence is larger then the 2.8W, but is smaller then the 6W we are now using. We have tried to engrave shaded photos with the 6W, but the quality and detail was not to our quality standards.
We built this unit to mainly speed up the 3D engraving process shown in our thread here:
With our experience, the 2.8W is good overall for engraving photos and cutting if you plan to do both.