ARM Cortex for hobbyists - Atmel or STM? Which IDE and tools?

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I use STM for all of my projects and it's the only MCU series I've used in anger. I learnt backwards: I used the libraries then worked out what registers they were poking. The documentation for the HAL library is zero. All you get is a PDF with all the functions and a function description which is basically the function name with the words in a different order. But, between the source and some documentation that tells you what registers to poke, you can get an understanding of the drivers and how to use them. It's taken me a good 6 months to become highly proficient with them.

PS. I'm curious as to other's experience with UART reception. In the STM MCU and drivers, you pretty much have to know the length of the data that will be arriving to get fast comms. Anyone have any other experiences?

There are some compiled html help files that you will love. They should be in drivers/Hal_drivers directory. Open them up and you can hyperlink around the document. It is a great way to quicly learn what functions there are and what arguments each function needs and what they are. Since you know the micro now its moot but pick a peripheral and just go through the compiled html doc. You will see what i mean.

I found the documentation really helpfull.  The user reference manual describes all the registers/bits. The include files are a good source also (driver directory).