On the Roblox wiki these two seem to have the same exact text concerning description features and implementation. There are no online answers to what differentiates them as of now. I was just wondering if anyone knows beyond this being some sort of redundancy.
OnServerEvent is for remote events. It can have multiple listeners and they don’t yield the thread when other side calls them.
OnServerInvoke is for remote functions. It can only have 1 callback and the calling side yields until result is received.
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I would also like to add this.
OnServerEvent is when the client directly sends data to the server.
OnServerInvoke is when the server returns data to the client when requested.
EDIT: Thanks for the correction incapaxx
u got that backwards 
the server returns data to client when requested. ofc the client can send their input through arguments to influence the result but yeah