Yeah, I'm in the same boat but I already have the external 1TB drive. I really want the 256 because my music and photos alone are 50GB. When I add in the installation of my apps, the 128GB is pretty much gone.
But since I can't afford the extra amount, I will have to live with the 128gb for now. I guess I can always upgrade it later.
I went the 128gb + 750GB portable external last gen. granted it was only USB2 but still, I felt being cramped with the 128GB internal was manageable but annoying. And since it was only the 11" (no SD slot) then I had no expansion options So I sold it to see what this gens 13 offers.
Now my dilemma is a 2011 13" 256GB vs a 2012 128GB (both are roughly $1100). With USB3 then I do get faster speeds and with an SD slot I can buy a it 64GB SD card but in the end I went with the 2011 model. The 2012 is nice but I'd rather have more internal space than faster external. (Or maybe a TB to uSB3 adapter will come out and solve everything)