The signal that you need one is specific: work conversation is happening in a channel that has no history you can search, or that mixes with personal life, and somebody is missing things as a result. Text messages are the usual culprit. When a decision made in a text thread cannot be found two months later, or when a new person joins and has no access to any of the context, that is the moment.
At that point the choice matters less than the discipline. All the established options do the same core things, and the differentiator for a small business is usually what your clients already use, because being reachable in the tool a client lives in is worth more than any feature. Several offer free tiers that limit message history, which is worth checking, since a limit on searchable history undermines the main reason you adopted it.
Two rules make it work rather than adding noise. Channels by topic rather than by person, so information is findable by subject rather than by remembering who mentioned it. And an explicit norm that it is not instant: an expectation of immediate replies turns the tool into an interruption machine and is the most common reason small teams end up hating it.
Whatever you choose, keep decisions somewhere permanent. Chat is where conversation happens and it is a poor archive. Anything that will matter in six months belongs in a document.
Whatever you choose, decide before adopting it what happens to the history. Free tiers commonly limit how far back you can search, which quietly undermines the reason you adopted the tool, and discovering that when you need something from four months ago is the wrong moment. Check the retention limit against how far back you would realistically need to look, and if the answer is longer than the tier allows, either pay for it or accept that anything important belongs in a document rather than in chat.
Agree where decisions live before adopting anything, because chat is where conversation happens and a poor place to find something six months later. Anything that will matter beyond the week belongs in a document, with the chat used to discuss it rather than to hold it. Teams that skip this end up searching message history for a decision nobody wrote down.