The M4 is better. However the prices are more likely to go down than up. And if it goes up it would be by a few pounds.
If you don't need one, then stick with what you have.
It's funny in a way. "wow £90 for 128GB!". Which is still way off hard drive prices

I paid more than that for my Vertex 2E 60GB not so long ago, but prices still have a long way to go. Not sure if SSDs could ever be competitive with big mechanical storage though, although given the complexity and precision engineering needed for high capacity mechanical drive, I wouldn't be surprised.