People prefer one of the other, they both have everything to offer (trees, steeps, alpine, cliffs, pillows).
I prefer Whistler because I find I get more vert on a pow day. You can lap Harmony and have interesting terrain every run with only one chair lift required. Peak and Red is a pretty fast cycle as well. Both mountains have good mid-mountain trees when it is storming.
On Blackcomb, lapping Spankys with glacier road, then Excel and Glacier chair takes a while (obviously it is worth it, tons of terrain in there, and it doesn't get destroyed quite as fast with the cycle time). Same with CBC. Chainsaw, two lifts again.
Blackcomb has a wider variety of backcountry access, but you can also get fucked very easily as it is mostly alpine.
Try to find someone to show you around, you can see a lot from the lifts, but some stuff (tree zones especially) are not as obvious.