We switched from Zendesk to Helpscout, at that time because their documentation portal feature didn't fit what we wanted (markdown edition and other things I forgot). The migration has been painless (all zendesk issues are imported to helpscout).

Apart from the documentation system, I personally prefer their UI (it feels more "modern"), the fact that a support conversation looks exactly like a very classic email conversation to your customers (no issue number in the email's subject, no specific signature or structure in the content etc), and Helpscout's team have always been helpful and nice so far. The API is pretty complete as well. We're using it only for support conversations nowadays, so I can't comment on the knowledge base features etc.