There are many, many differences - all laid out below - but the core of it is this: Draftboard is a product build entirely for finding warm intros, while Sales Navigator is a prospecting tool that has added in some limited functionality around warm intros.
Focus is everything, and it shows. Here's the full list of things that Sales Navigator struggles with:
No scoring of Relationship Strength
Draftboard calculates a score for each Connection/Target path so that you can easily focus your efforts on the highest likelihood paths. Sales Navigator offers nothing like this - you'd have to visit each mutual connections profile and visually scan it to check any overlaps (and how they compare to other paths).
In same cases LinkedIn offers filters (e.g. "Past Colleague"), but this is a binary filter that contains no nuance. Whereas Draftboard takes into account the length of overlap, the recency of overlaps, whether there have been multiple overlaps, respective titles & departments, relative seniority, etc.
No rhyme or reason to the order that mutual connections are displayed (or at least it's not shown to the user)
No CRM to close the intro request loop:
You can't mark when you've asked for intro
You can't mark when someone declined to make the intro
You can't see if an intro is in progress
You can't see if an intro was made
Because of all the above, you may end up asking multiple people for the same intro - a waste of their time and your professional capital
No support for customized message templates
You have to write each message from scratch
No dynamically generated templates that you can apply easily based on the type of connection you're asking to make an intro (e.g. investor, advisor, happy customer, friend, etc)
No way to prioritize your best connections ("Supporters" in Draftboard) in the mutual connections results
No Connections View - i.e. no way to at a glance see which of your Targets your Connections have a path to (e.g. it's very helpful to send your investor an email saying "Hey - are you able to make any of the 9 intros listed below?")
You can't import a Target (lead/prospect) list via CSV to Sales Navigator - so if you're using Apollo, ZoomInfo, Amplemarket, etc for prospecting, you're out of luck (unless you want to build the list again manually on Sales Navigator)
You can only see the names of 2 mutual connections per Target on the Sales Navigator results page - to see all of them you have to hover over 'mutual connections', wait for the tooltip to show, then click on the link provided to open an entirely new tab.
In general, everything on LinkedIn requires opening new tabs - you'll have dozens open in no time for everything from sales nav profiles, linkedin profiles, linkedin messages, sales nav inmails, email windows, mutual connection screens, etc etc.
TeamLink only supports 1st degree connections of your team members - Draftboard supports 2nd degree (~1,000x difference)