One of the big advantages with STM32 is that they have a range of MCU’s from the small M0 series to the more powerfully M7 series. Changing MCU to scale up/down is very easy. The MCU’s will have some differences in pin usage and register usage, but ST have been decently good at keeping their stuff as compatible as possible.
Pricing is another issue, buying through a distributor it is not that much difference between a M4 and M7.
The differences between these two are huge. H7 runs at 480Mhz, have 1Mb SRAM, 16 bit ADC channels etc. This is of no consequence if all you want to do is to blink a led. I have standardized on STM32F405 and that alone has far more power than I need for most parts.
STM32CubeIDE have also changed + H7 comes in a 100 pin package. The ADC channels combined with 1Mb SRAM and M7 opens for interesting data acquisition techniques so I have decided to try out the H7 on a Analogue Data Acquisition Hat. In fact I can use my existing nucleo to test out SW concepts, so I don’t need to start with HW in this case.