PicoPower - I can't tell you the specifics, as comparing power consumption is a bit difficult (you need to figure out which peripherals you need running, whats the system clock, input voltage, sleep/wakeup regime etc and calculate from there), but they do have STM32L0 series of ultra-low power based on Cortex-M0+.
As to why do I like ST:
1. They are very easy to design boards for. Each IO pin on the Cortex micros maps to up to 16 different internal functions. That gives amazing flexibility from the board design perspective.
2. They have fantastic long term migration paths. You can directly go from Cortex-M3 to Cortex-M4 to Cortex-M7, while, in many cases, retaining package compatibility.
3. They have very good design validation. Their errata sheets are quite shorter than what I remember working with other manufacturers.
4. They have a huge amount of useful IO, such as timers (I have one design using over 15 timer channels) and a quite flexible and powerful ADC architecture.
5. They probably have the widest selection of Cortex microcontrollers in the industry.
There are downsides, too; I generally find their peripherals less advanced than Atmel's (regular CAN instead of CAN-FD, USB inferior to Atmel's, I2C that's really sucky and prone to errors, slower clock and no dual precision FPU on the new F7 family). There was also some story with ST's 2MB flash parts, being not really usable - but I'm not sure what was that about.
Overall you can't go wrong with ST. The long term migration ability and board design flexibility are well worth the downsides.
Coming from an AVR, you're immediately going to get way more memory, much higher clock frequencies, ARM CMSIS libraries (fun if you have any math/DSP going on) and more industry standard tools.