Shall we create a new branch or a new repo when create a new SW variant in gith/github?

Question

I am developing the embedded SW, and the project now has a new variant which share about 90% same code but can not be merged back in the future.

Then shall I create a new branch in the old repo, or just create a new repo for the new variant?

And more background: the 2 variants shall have similar requirements, for some modules they are totally same, few modules are not.
And when new feature comes, may diff on different variant or have different implementation schedules.

What I want to obtain is that similar code changes on 2 variants can be reused/shared which means do not have to write the same/similar code twice. Not sure if this is possible.