Differences between Cortex-M4 Cortex-M7 and Cortex-A5
Stress some are cheaper. Cortexes are just microprocessor IP cores that semiconductor companies buy from ARM. The CPU core is one of many cost factors in a microcontroller. 

Comparing A5 to M something isn't fair. They were designed for totally different customer classes, and the fact that some A5s are cheap is an effect of 


economics of scale and
the fact that you pay for the silicon quality, and hence, reliability you get


The exact core architecture of the microcontroller you're planning to use is, I daresay, a secondary feature, especially with arms, since there's simply mature C compilers, so you normally select by the peripherals you need and the compute performance and things like overall power efficiency.

What's the benefit of having a very watt per megaflop-effective core when you're in deep sleep mode 99.95% of time, and the smaller microcontroller would allow you to shut down the external oscillator but the efficient bigger one won't?

What's the disadvantage if your processor core actually uses ten times more energy, but your system's power consumption is defined by the heavily loaded output of a PWM unit?

What good is a high-performance MCU when it needs much better quality power supply?

What about not having something with a large BGA pinout that makes two-layer boards impossible?

Why throughput when what you need is guaranteed interrupt latency, instead?