Also consider shield usage. Some shields are not compatible with the Mega because they depend upon special-function pins that are in a different place. For example, the SPI and I2C/TWI pins are in a different place on the Mega than on the Uno. It is possible to work around it through breakout shields, cross wiring, etc. but it is a consideration.
The Gadget Shield
: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons