Great question. I will answer it by saying this.
Most people will tell you that the Bank has one chance better than you in every 37 spins. This is correct if you only play 1 number "en plein", or straight up, but if you play on an even chance the Bank has only half this advantage over you , or one chance better than you in every 74 spins, the reason being that when your stake is on an even chance, and zero appears, you do not lose your whole stake, but only half of it: the player always having the right to take half his stake.
You would think that it would be fairly obvious to most people if you put a unit on the first 18 numbers straight up instead of putting 18 units on Low "Manqué", but in essence you are doing a foolish thing; for when zero comes out as it should do once in every 37 spins, you would lose the whole 18 units, whereas if you had them on "Low" or "Manqué" you only lose 9 of them; in other words the player who plays on the numbers is quite sure to lose, on average, 9 units an hour more than the player who plays the Low or Manqué even chance.
This answer is also cited in the book "Ten Days at Monte Carlo at The Bank's Expense" pgs. 5-6. I agree with this philosophy and as a player I have also experienced it both ways and it holds true in my experiences of playing the game.
Now in American Roulette, there is no "en prison" rule for zero and there is a double zero as well. But the pay out is 35 to 1 on the numbers not 36 to 1 so again I would bet 18 units on an Even Chance instead of 1 unit on 18 numbers.