There's a KB article on the subject -
http://software.intel.com/en-us/articles/mkl-ipp-choosing-an-fft/MKL and IPP are distinct and different libraries, although in the case of FFTs and VML they do share optimized kernels at the lowest levels. There are likely cases where MKL will give better performance and others where IPP will lead. Generally MKL focuses on performing well for mid to larger sized problems common in HPC; IPP's focus is smaller sizes common in the multimedia, data processing, and communications areas.
Both MKL and IPP provide optimizations fordifferent IA processors.
As for your last question, much depends on how you intend to use the libraries longer term. You should be able to download free 30-day evaluation copies of both and see which one best suits your needs.
-Shane