I recently changed my front brakes (K11C 5 door 1348cc (1.4)). Depending on what website I went on, it said I could use vented or solid. I think there was a difference of 4mm in diameter between the two and £40. In the end I went for solid as it had solids on it. With trade card, they were £30 from halfords for discs and pads.
Any noticeable benefit, given the relatively low speeds these cars can go, will be from pad upgrades, braided hoses and a fluid refresh with some quality brake fluid.