What is the difference between ARM Cortex M7 Cortex A53 (or A17)? Why one will choose one over the other? To what kind of applications does each of those is targeted for? Which one will be more suited (and why?) for wireless communications and video?
The Cortex M series all fit into the microcontroller family, that is they are specifically optimised for smaller, specific, time-sensitive applications. Because they have a more specific job, they typically don't have as much memory.
The Cortex A series are more of a general-purpose embedded type core, that you would find in a smartphone or a tablet or other SOC's. They typically favor higher performance and functions that are more suitable for running an OS such as Android.
Note that both of these are just families of processor cores, they aren't the actual processor bundles including memory and device interfaces. You didn't really provide much information about what you're looking for, but if I had to pick a hardware platform for rich multimedia and streaming, it would be an SOC that had the A-series core in it for performance reasons.
Generally those are the names of two completely different microcontroller architectures, where Cortex-A is rather microprocessor than microcontroller. Specifically Cortex-M7 is the new Cortex-M architecture designed for the purpose of Industry 4.0 standard and Internet of Things. Whether microcontroller or microprocessor is a very general question and depends of whether you are building an embedded system or portable computer such as smart phone, tablet or digital camera. Both of them are suited for wireless communication and video and I will advice you before you choose one of them to test each of them on an evaluation board. For example Firefly RK3288 for A17 and STM32F746G-DISCO for M7 will do the trick.