Compare Entity Framework 6 and Entity Framework Core | Microsoft Docs
EF6 has been around for longer, and supports some things that EF Core doesn't. On the other hand, EF Core supports some things that EF6 doesn't. And all future development seems to be targeting EF Core.
Which version you pick will depend on many factors which we do not know, so we can't tell you which one would be the best fit for your project.