You can do green screening and a gun flash in one program if you have VisionLab, but if you had ELab and CLab you'd have to do the green screening in CLab render it, and then load that shot into ELab and add the gun flash. Other then that (and a few filters exclusive to VisionLab) the effects work would be the same.
VisionLab also has a commercial license, which means you don't have to credit FXhome when you sell work you've made with the program (you couldn't credit them if you were making a commercial or music video or something along those lines)
If you are on a tight budget ELab Pro + CLab Pro is the way to go, but if time is of the essence and you have a nice budget VisionLab is great.
take a look at this thread
it has a few links to different topics about this