The 5K is well, well worth it and will likely see just spec bumps that will increase performance and such by around 10% next year. However, the 4K iMac is not worth getting as it will be updated to Skylake next year with a much better iGPU. As for a redesign, not likely, but in reality, no one knows if they will ever get rid of the chin or give us a touch screen iMac (though not likely)