In Australia, because all games need to have been rated by the OFLC, Xbox Live Indie Games are not available.
So, this thread makes me sad. At least PC gaming exists. And XBLA is pretty great at times; Super Meat Boy, the awesome Double Fine games like Iron Brigade, what have you... but it's all official and approved and stuff. Sometimes I just wanna play a game that feels like it was made in an afternoon by a closet programmer... but... on Xbox. IS THAT TOO MUCH TO ASK ;-;
...that said my answer would've been XBLA anyway cos at least it has quality control to some degree
I havent really played much of either, but I remember having Marble Blast Ultra and the Original Doom on Arcade, so im going to have to lean towards XBL Arcade.