I'm not sure about the guru part... Your Credit 41 is a glyphosate (like RoundUp). Different brands have different surfactants loaded in them, makes it stick to the leaves better. That's why some seem to work better than others. It is non-selective so it will kill most anything that has not developed resistance.
Mike's 2,4-D shouldn't kill most grasses, stunt it a little. Stuff kills most broadleaf plants. Once the grass comes out of dormancy I use the 2,4-D to spot spray as needed.
Crabgrass is a whole different animal. Need to kill it with RoundUp early in the spring and get some pre-emergent down. My preference is a product called Prowl H2O. It is some nasty yellow stuff. Mix in some blue spray dye and 'paint' your lawn green in the spring. Seems to keep my lawn fairly weed free.