A particularly nice feature of the AVR/328P is the 100k single-byte-writeable EEPROM, and few ARM variants seem to have something equivalent, though we have found a 50 cent device that is close and are doing some preliminary design around it.
I've been using the STM32 series micros for a while now and I'm very impressed. One project uses a STM32F303xC which has 256k ROM, 48k RAM, I'm using an external SPI NOR flash of 8Mbytes (W25Q64FV) to replace the EEPROM. So far I'm using 4 out of the 5 UARTs - 2 with hardware flow control, 10 DMA channels, 10 of the timers, SPI, I2C, DAC, 14 of the ADC channels and I can still put it to sleep with a current consumption below 50uA!!
Just having an decent IDE with multiple real-time breakpoints has improved productivity 3-4 fold.
The STM32F103 boards that I started with are a couple of dollars and the USB debugger is the same price. Even going up to a STM32F3 series Nucleo board which has the debugger as a "snap-off" PCB gives you change out of $20.
Steep learning curve but you'll be ahead within 6 months.