The verdict
If you need a steady stream of on-brand visuals, not only decks, pick Canva. The brand kit keeps a team consistent without a design review for everything.
If your main need is turning notes into a structured deck, document or one-pager quickly, and you are happy to edit the result, pick Gamma.
For most small B2B teams I'd pick Canva, because it replaces a patchwork of design tools and its free plan goes a long way. Many teams will still use Gamma for a quick first draft. Never send a Gamma draft to a client unread, since the copy is generic until a person edits it.