The verdict
For most small teams, I'd pick Plausible first. It is easy enough that everyone reads it, and it answers where traffic comes from. It is the right tool until you can no longer say which campaign created customers.
I'd pick Spectacle when you are B2B SaaS, product-led or long-funnel, spend real money on paid channels, and finance wants numbers it will accept. They are not alternatives so much as steps. Spectacle does not replace your CRM or web analytics, and its output is only as good as your deal stages and tagging. If your CRM is messy or the sale closes in a day, a simpler set-up is enough.