I've got 5 Cancer CW's and two Gashers. My scale isn't auto-calibrated but a 50g weight reads as 50.05g, so subtracting .05g from the weights I've listed should give fairly accurate weights.
I've included the conditions of each, i.e. where they came from and whether or not they have stickers.
Cancer.
1. Dark Cancer CH120SF, Stickers, 3.14g
2. Dark Cancer CH120SF, Stickers, 3.17g
3. Dark Cancer CH120SF, No Stickers, 3.18g
4. Unknown - Yellow, No Stickers, 3.14g
5. Super Deck Set, Stickers, 3.11g
Gasher
1. Dark Gasher CH120FS, Double Stickers, 3.37g (Put stickers over the other ones.... I was a newbie k?)
2. Dark Gasher CH120FS, Stickers, 3.20g
While it seems gasher may be slightly heavier, I do not think this is a significant difference to draw conclusions from, especially with me only owning two gasher CW's. Even if it is, the difference would be smaller than that of putting stickers on a beyblade, so there wouldn't be a practical difference in performance.
I'm also quite sure the "Hasbro dark is better" thing was entirely based upon those incorrect set of weights, as my hasbro and TT dark wheels didn't show any real separation in weight ranges.