LaTeX3 versus LuaTeX: which is the future? Statements by LaTeX3 team members. 14 years ago
I guess both new LaTeX3 syntax and the Lua part will be hidden if I'm a casual user. So I will indirectly benefit from both, while the package and class authors will hopefully use the new capabilities.
And why not working with a LuaLaTeX3 - the best of both worlds can be used.