I'm trying to decide if its better to buy Mana Regeneration runes or Flat Mana runes. I have already decided that if I buy the regeneration ones, it will be regen at level 18, due to the fact that after level four they regen more mana. Flat Mana runes = 101.25.
Mana regen at level 1 = +.72/5
Mana regen at level 2 = +1.44/5
Mana regen at level 3 = +2.16/5
Mana regen at level 4 = +2.88/5
Mana regen at level 5 = +3.6/5
Mana regen at level 6 = +4.32/5
Mana regen at level 7 = +5.04/5
Mana regen at level 8 = +5.76/5
Mana regen at level 9 = +6.48/5 and so on

Thank you in advance.

I think Flat is better since mana regen is most important early game and later game your champion can handle the mana easily.