Which is faster, Blendshapes or Non-Uniform Scaling?

So, I have a theoretical system in which the player can stretch and squash parts of a mesh. Like, lets say you have a corner piece for a ceiling, they can make both parts of the piece longer and shorter. Now, I could make these two separate meshes that after you're done changing it, it gets combined into a single mesh with Mesh.CombineMeshes.

There's an alternative that may be easier for a less-experienced coder like me. I could use Blendshapes/Morph Targets to do the same thing. I remember reading some Spore (A game with many user-created objects) design documents, and they said that morph targets were too slow for what they wanted to achieve.

So which should I do? Both are capable in Unity as far as I know, but I've heard bad performance issues with both.

Edit: I just realized; if you combined the meshes, would the scaling even stay?