Id go with a teensy++ (or even a Teensy) instead of either one of those.http://www.pjrc.com/teensy/
The Teensy++ is cheaper than many places sell the Uno board and has lots more resources than the Uno.
(even the Teensy has more resources than the Uno)
The Teensy++ has 128k of flash (half the 2560) but I don't think code space will be an issue for this project.
It has the same RAM as the 2560 and 46 instead of 54 i/o lines (should be more than enough).
To me the big advantages are lower cost and smaller size.
Teensy boards are tiny and much lighter than either the Uno or Mega board.
To really save weight and some cash, you could even build your own led matrix
to scroll some text. See this project:http://www.nerdkits.com/videos/ledarray2/
You could keep it small and use some very light weight foam board to hold the leds.
If you paired this with a teensy++ (even the smaller teensy would work) it could
be fairly small and very lightweight.