I have both that I have used with a 5000lb tractor (4.2K or so with rear blade on)
A front blade on a tractor this size is pretty much worthless with a couple of exceptions. The front blade I have that replaces the loader bucket can be angled in a couple of directions by "tilting" it, which unfortunately also changes the angle of the cutting edge. Even so it is good for doing light landscaping of slopes of ALREADY loose, rockless ground. I hate trying to look backwards and wreck my neck (talk to old guys with lots of hours) using a rear blade to angle slopes. The front blade will just throw the tractor up off the ground if it hits anything remotely solid. If you have a "real" front blade, one that mid mounts you'll do better. For out and out digging a rear blade is best, just as a rear anything seems to work better. I use the front blade going backwards, tilted forward it is great for just removing sod and leaving a flat surface for gravel. It also works great (with it's "legs" on) to move snow. It's not a snow blade it weighs probably 800 lbs. Also works beautifully for spreading gravel.
I just need to figure a way to drive my tractor facing backwards! /w3tcompact/icons/smile.gif
Any heavy work is better with a rear box scraper or the local rental yard D3 cat.
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