The difference is relatively simple, a 6 way blade, is best suited for fine grading, the 4 in one, is more utilitarian, pick, load, though when you open the bucket, you have essentially a dozer blade, you don't have tilt or angle, so its a lot more work, planning or what have you to start a sloped cut or grade something that is not flat, put a windrow on one side, get the machine tilted, say for ditching or what have you/ Way back when, this was the method, you can see much of this in the instructional portion of LeTourneau's dozer kits from the 40's and before. Though there was manual tilt and angle models others were straight push arms, and thats what you have with a 4 in one bucket, typically, as its on a set of loader arms. You can stump small trees with a grading tractor and 6 way blade, push same off to clear an area, ditch, grade, cut, or what have you. If you have a lot of material to move or load out, well the 4 in one may serve you better. Like has been mentioned a rubber tire backhoe, and this country is loaded with used ones, is also something to consider when figuring on 1 dedicated machine. Ideally, a backhoe with a 4 in one bucket, and backhoe bucket thumb is an extremely versatile machine to have, add a dozer with a 6 way blade, not much you can't do within the confines of each given size and power.
Realize that a 6 way blade is a nice feature, but if you have large rocks, trees or what have you, better to have a larger dozer with push arms on trunnions attached to the track frames, those will tolerate the heavy forces at work and are designed for same, unlike a 6 way. Small trees are fine, tilt, angle down, cut the roots, cut around them, and push over wont hurt a thing.
I was on a D6D Cat going back to '93 or so, and was pushing off large pieces of blast rock in a fill area, tilt cylinder bore had a some wear, and I found out how strong it was pushing from one side a little, this was a straight, push arm blade with only tilt, broke the bore and pulled the cylinder free. Lot of power to those tracks sometimes, equipment mechanic said it was due and saw from what we were doing, should have had the D8K in there.