Euthanasia has more support, but legalizing marijuana has far more potential politically for three reasons.
1) Other countries already have euthanasia policies in place. This is not the case for marijuana. No one has legalized pot. If Canada were to legalize pot, the amount of international attention would be enormous. The Conservative tough on crime agenda would be starved for oxygen.
2) Not only does this have the support of huge numbers of people internationally, the issue is pregnant in ways that euthanasia is not. The huge cost of the war on drugs is straining budgets. Marijuana prohibition is quickly loosing legitimacy in the Western world, is quickly being rendered untenable by emergence of loose medical marijuana laws (e.g., California) and is feeding corruption and drug related violence elsewhere, most notably Mexico.
3) Euthanasia like a abortion rises extremely complicated philosophical issues in ways that marijuana legalization does not. Deeply complex issues relating to personhood and viability abound. Marijuana is much more like SSM in that opposition arguments are so bad that that it mocked as a form of madness, reefer madness. This matters a lot. The Liberals will benefit from having the Conservatives trout out the same dumb arguments months on end.