You did not specify, but from the HF price I am assuming you are talking about stack dado blades and not wobble blades.
I have two sets of stack dados. One from HF, the other from Delta. Both are 8" sets. My experience is the Delta produces a flatter dado cut. However, the HF seems better balanced (less vibration) and produces straighter cuts. I find myself using the HF more, because you rarely see the bottom of the dado once everything is assembled.
The comments about the flatness of the bottom of the cuts has already been discussed in other posts.
If you are referring to variations in the width of your dado, that can be caused by a few factors.
1. You could have a bent spindle on your tablesaw, in which case you should see similar issues with a single blade.
2. You may have the two outside blades of your stack dado reversed. With most stack dados, the blade teeth are cut on a bias. So, there is a defined outside to the blade. If you were to look at your dado blade from the edge, the teeth of the two outside blades should slant toward the inside of the dado stack.
3. Also, it is possible (although not likely) you may have chipped some of the teeth or lost carbide tips. Or, bent a chipper blade.
BTW: I doubt it matters much, but I do most of my dados on my radial arm saw. The only dados I do on the table saw are those that are longer than the reach of the radial arm.