Tough call. Both awesome games. Both awesome single player campaigns.
But I liked acing the skirmishes in Valkyria chronicles more then the side missions in Mass Effect.
The graphical style of Valkyria chronicles is something special, it will still look awesome 2 generations from now. I also felt a little more connected to the characters in Valkyria Chronicles then in Mass Effect.
Valkyria chronicles wins.
Oh I missed the killzone 2 vs Halo 3 vote. Just as well, I did not play halo 3 and Killzone 2 was pretty but average.