These apps serve different purposes, so I don't think one can really answer the question without knowing the use-case.
My suggestion is if you only care about webmail, then Rainloop wins. It's quicker, nicer interface (IMO), intuitive, etc. If you however also want to manage a calendar and contacts, then SOGo is likely the better one as Rainloop does mail and contacts only, no calendar or anything like that. And even for contacts, it's differently managed - Rainloop simply stores them, but SOGo "hosts" them so they're also accessible via CardDAV protocols into apps like Contacts on macOS or iOS, Calendar on macOS, etc.
Basically, Rainloop is a webmail client whereas SOGo is what's called "groupware" as it will sync contacts, calendars, tasks, etc. in addition to the mail client role.
If you elaborate on your use-case, then people can probably give you a more accurate answer. Hopefully the above helps in the meantime.