Each has its advantage.
Traditional Ghillie - PROS: nothing diffuses light better than a traditional ghillie. Easy to use and make or add more material. CONS: burlap is smelly, snags on everything and it is hot and heavy - look out of it gets wet
Newer mesh Ghillie (like Kill Suit and newer Cabellas) PROS: very light weight, easy to fit in a pack, does not snag on everything and you can just throw it in the washer and they are CHEAP, and they can be worn over any combination of material CONS: can't easily add material, does not diffuse light as much
For me I love wearing a traditional ghillie when it is cool enough and there is not a lot of underbrush - but I find myself wearing my various Kill Suit combinations 95% of the time. The sapper suit is all you really need as it covers down to mid thigh. BTW - another advantage is that the Kill Suits are much easier to climb a tree with although I usually put mine on once I am in the stand. Yes the right Ghillie is also very effective in trees.
Here is the difference between reg camo and ghillie. Regular camo was invented to help you blend into environments and make it harder to outline or see you. Ghillie suit camo was developed/perfected to remain undetected by an enemy actually looking for you.
I cannot prove it but I get away with a lot more slow movement with a ghillie then I ever have with any camo including the open patterns like predator and ASAT. I've had deer and elk see me draw my bow with the ghillie but they seldom blow out of the area and very few became highly alerted. Most just noticed me and then shrug it off as a non-threat. That has been my experience withe the Sapper/kill suit as well.