The two have converged a lot over the last three years — both ship the obvious sales-engagement primitives (sequences, dialer, recording, conversation intelligence). The actual day-to-day rep experience tends to favour Salesloft on simplicity; the data, reporting, and AI surface tend to favour Outreach.
For larger orgs, the deciding factor is usually which one your existing tech stack and admins are familiar with — switching costs are real either way.