Hi Alexander,
I think that the best IDE for you will depend on the needs and requirements of your project, you need to consider that KDS is a newer IDE and slightly less "mature" than IAR, it is free of charge and unlimited, while the free IAR version has a code size limitation (16 KByte for ARM Cortex-M0+).
On the following links you can review the features of both IDEs:
Kinetis Design Studio Integrated Development |Freescale
IAR ARM
Hope it helps!
Best Regards,
Carlos Mendoza
Technical Support Engineer
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