It's just as small, and it's easier to use, so you can do more. The Trinket M0 has swapped out the lightweight ATtiny85 for a ATSAMD21E18 powerhouse. We've taken the same form factor we used for the original ATtiny85-based Trinket and gave it an upgrade. It's our lowest-cost CircuitPython programmable board! Perfect for when you don't want to give up your expensive dev-board and you aren't willing to take apart the project you worked so hard to design. We wanted to design a microcontroller board that was small enough to fit into any project, and low cost enough to use without hesitation. The bootloader does not affect RAM usage.The Adafruit Trinket M0 may be small, but do not be fooled by its size! It's a tiny microcontroller board, built around the Atmel ATSAMD21, a little chip with a lot of power.

The bootloader on the Pro Trinket use 4KB of FLASH so the maximum sketch size is 28,672 bytes.

The 3V Pro Trinket runs at 12MHz not 16MHz so its a bit slower.

This is to keep the Pro Trinket small and inexpensive, you can use any FTDI cable to connect to the FTDI port for a Serial connection

There is no Serial-to-USB chip onboard.

You cannot plug shields directly into the Pro Trinket.

The onboard 3.3V or 5V regulator can provide 150mA output, not 800mA out.

Use interrupt #1 on pin #3 instead with attachInterrupt (1, functionname, condition)

That means you do not get access to interrupt #0.

The only changes you may have to consider when adapting Arduino sketches are: The Pro Trinket 3V runs at 12MHz, which we suggest for 'power users' who want a 3V Arduino-compatible and don't mind its a bit slower at 12MHz. The Pro Trinket 5V runs at 16MHz, just like the Uno. It's the 'classic' Arduino chip! So you'll be happy to hear that not only is Pro Trinket programmable using the Arduino IDE as you already set up, but 99% of Arduino projects will work out of the box! The Pro Trinket uses the Atmega328P chip, which is the same core chip in the Arduino UNO/Duemilanove/Mini/etc.

Adafruit trinket 5v arduino ide