Adafruit Pro Trinket - 5V 16MHz combines everything you love about Trinket with the familiarity of the core Arduino chip, the ATmega328. It's like an Arduino Pro Mini with more pins and USB tossed in. Adafruit Pro Trinket - 5V 16MHz designed for projects in which users need more pins, more flash and more RAM. It has 18 GPIO, 2 extra analog inputs, 28K flash and 2K of RAM.
Like the Trinket, Adafruit Pro Trinket - 5V 16MHz has onboard USB bootloading support and a MicroUSB jack. It also has Optiboot support, so you can either program your Adafruit Pro Trinket - 5V 16MHz over USB or with a FTDI cable just like the Pro Mini and friends.
Adafruit Pro Trinket - 5V 16MHz uses the Atmega328P chip, which is the same core chip in the Arduino UNO/Duemilanove/Mini/etc. However, it uses 3V logic and 16MHz clock which means it's good when you want a lower-power and lower-voltage setup. Adafruit Pro Trinket - 5V 16MHz is programmable using the Arduino IDE as you already set up, and 95% of Arduino projects will work out of the box!
Here are some things you may have to consider when adapting Arduino sketches:
Pins #2 and #7 are not available (they are exclusively for USB)
The onboard 5V regulator can provide 150mA output, not 800mA out
You cannot plug shields directly into the Pro Trinket
There is no Serial-to-USB chip onboard. 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. The USB connection is for uploading new code only.
The bootloader on the Pro Trinket use 4KB of FLASH so the maximum sketch size is 28,672 bytes. The bootloader does not affect RAM usage.
Adafruit Pro Trinket - 5V 16MHz - General Specifications
ATmega328P onboad chip in QFN package
16MHz clock rate, 28K FLASH available
USB bootloader so you can program it with AVRdude and/or the Arduino IDE (with a few simple config modifications).
Has headers for an FTDI port for reprogramming
Micro-USB jack for power and/or USB uploading, you can put it in a box or tape it up and use any USB cable for when you want to reprogram.
On-board 5V power regulator with 150mA output capability and ultra-low dropout. Up to 16V input, reverse-polarity protection, thermal and current-limit protection.
Power with either USB or external output (such as a battery) - it'll automatically switch over
On-board green power LED and red pin #13 LED
Reset button for entering the bootloader or restarting the program.
Works with 99% of existing Arduino sketches (anything that doesn't use more than 28K, and doesn't require pins #2 and #7)
Adafruit Pro Trinket - 5V 16MHz - Technical Specifications