This is a tough one. From a Canucks perspective, yes...you want the big goal scoring power winger. But...you also have the weigh the contract situations, where it's not altogether clear how likely Tkachuk is to want out of Canada like his brother, and you're simply getting less of that "value deal" either way before the next contract has to be negotiated.
The other aspect is...i think Thomas is the better player in a vacuum. Tkachuk's edge is in that he's in some ways a more "rare" player as one of the few remaining quasi-power Forward types. But Thomas is better, more versatile, and more well-rounded.
I think ultimately, i probably just take the player i like better with the better contract situation, and just try to find a bit of "size" and "physicality" somewhere else.
Whether that'd be the Lotto-line + Thomas anchoring a 2nd unit. Or Thomas centering Pettersson and a Miller-Boeser duo on the other...that's not something i worry about patching the "3rd wheel" on either line too much. It also gives more option to just run impossibly 3 deep that nobody could match with all three carrying a line. And more importantly, it'd give the Canucks a lot more of the speed they kind of lack in the Top-6, as well as far better defensive play, more ability elevate linemates than Tkachuk, and more contributions to things like the PK and RH faceoff options that do also matter.
So yeah...i'd take Thomas. Even if Brady is the "easier" direct plug in to a current hole in the lineup.