I'm agree with Juice_Lizard
The points for the Arduboy: light, cheap, very good advices to make hardware things or to make your program better.
Observations: Very few memory but enough to do good games as you can see in their library but you have to optimize alot memory use for big games
The points against Arduboy: No Python usable atm, you have to write a game each time in the memory as you don't have ability to choose game (it's partially tru as now, you can add some hardware and software to be able to upload more games and have a menu to choose it but it's not in standard and not available in the package when you buy it)
The screen have good definition but it's a black and white screen (users have invented technics to make some gray scale on it too)
The points for the META: Academy is more easy to be followed even it's can always be better. Langage is french and english (Arduboy is mainly english and a little japaneese).
META can be programmed in C as Arduboy but you can learn Python on it too.
It's a color screen, with a bootloader with a gui to choose game to launch
Observations: ATM where is not alot hardware tutorials but this will change soon and there are some videos (in french atm) to learn how to use hardware with it.
Points against: The main is the price even it's go down and there is sometimes good promotions (mainly when you buy 2 or 4) and with the 'Parrain' system wich let you buy it a little cheaper.
All comments upper are only my vision of the things some other could have another vision. Me i like the both and ada fruit have to be consulted too as there is some good things too.