The mega has 3 serial "ports"
No, the Mega has four hardware serial ports, the standard serial port and three extra.
As far as application power goes there is no fundemental differences between the Arduino and Seeeduino Mega boards. The Seeeduino just has a few extra features like manual switchable automatice reset, manual switchable power source. It also has 16 extra I/O pins brought out to solder pads, they aren't supported by the Arduino IDE core but you can use them via direct port access. Also the Seeeduino board is smaller (same form factor as the standard Arduino board) and uses smaller USB and external power connectors. Also the board's reset switch is accessable even with a shield installed.
I bought the Seeeduino version so I am not impartial but both Megs are great offerings.