This is just my opinion. Honestly snowball mode is so much fun and I find it much more enjoyable than spikerush. It actually feels like a casual and relaxing game mode and you won't find people tryharding on it. This is not to say spikerus...

I used to think that it is based on whether you outperformed your current elo in a match. But today I (Silver 2) played a match and got Match MVP in a lobby with 6 players who were Silver 3 and Gold but still didn't get any performance bonu...