My reasoning is a bit shallow here. In my first TW2 playthrough I went heavily into alchemy and I'm still convinced that it's the strongest of the three in that game. But in TW3 I'll want to make use of the visuals, and I don't see how Alchemy will make fights look cooler than focusing on Signs or Swordsmanship with all the cool upgrades they offer. It also doesn't offer many active abilities. So while my numbers were noticeably higher with the Alchemy build, battle still played out like a low-level Geralt. Magic and sword fighting all looked very basic - they just dealt a lot of damage.
Yeah, it offered more mutagens and of much better quality as well as improving your tools, but mutagens are passive and tools for me are support rather than main.
So swordsmanship it is (for the first playthrough). I'll be disappointed if there are't too many active abilities to unlock and almost everything is just passive upgrades in it.