It depends. Historically if the US does something Canada does, the US companies squash the Canadian ones. That means if you’re invested in Canadian growing companies, US legalization would tank your investments.
However, the US right now seems to be aiming more at a State by state legalization procedure where the federal government will not legalize but also not meddle in state affairs. Without federal legalization, US growers cannot get loans, and are not required to meet federal regulation by the FDA so they can not expand at the rate of Canadian companies can. Since Canadian companies have said they will not sell to the US without federal legalization, increase demand for pot in the US from say 2/3 of states legalizing it will only increase the US’s dependency on Mexican illegal Cartels.
The legalization trajectory the US is on will save lives from being ruined over the drug war for retail corner dealers, and users but will not raise Canadian pot stocks or increase the health safety of the pot being smoked. It will also not solve the problem of black market cartels.