It depends what you have in mind once you get the ka.
I don't think there are many options open for tuning as Ford only put that 1.6 8V into the Sportka.
I've got to admit that I like the look of the KA's especially in IMP' Blue, and I'm sure that they'll return a little more MPG, and they're a lower insurance group too so that will bring some more money in!
However I personally would keep the Milly as it has the performance, the looks and the seats are much better too