Reason for Rennes' optimism may be that Marseille will not be highly motivated to win the away game, but on the other hand, although Marseille's position in second place is quite solid, they need to score at least 2 points to protect themselves from an attack from Nice. Now I think that this game will end in a draw. This would be the most logical result.
BTTS may be just the right bet to make -- Rennes scoring every game since April, but I'd probably bet on the losing team live, as soon as first goal is scored -- outright if an early goal, handicap or double chance if second half.
Impossible to choose, my form brain normally picks Marseille in this case (and biting back from the Feyenoord-induced exit) but then also says time for Rennes to do something special.