In your later levels, the mob will usually get to you in time anyways, so a higher level spell may result in killing faster, yes, but will consume much more SP and have a much longer cooldown.
Secondly, MM1 and FB1 spam are generally recognized as filler spells. You will get an excellent spell named Magic Blast at level 49, which will become another one of your spammed spells. It has good damage, low SP consumption, and fast cooldown. You will generally want to have around two to three filler spells; MM1 and FB1 usually suffice, but Magic Blast also become a useful spell in your arsenal.
Thirdly, in later levels, people will generally prefer to use AoE spells, such as Frost Nova and Inferno, to kill multiple mobs at once. Consequently, you will find yourself more of a DD slave than anything, without any real need to spam the upper tier spells. The lower level spells will suffice to eliminate any mobs that survive your barrage.
In the later game, you will have 7 single target spells to use. One lower level elemental (Magic, Ice, Fire, and Lightning) along with one higher elemental (Magic, Ice, Fire, and Lightning). I say 7 because your Ice spells will share a cooldown with one another.
Trust me when I say this, but there is absolutely no need to invest into upper level spells. The only single target active spell I urge you to upgrade levelwise is Fire Ball. Magic Blast is optional—I personally did it—but this one I merely suggest. Fear and Drain Mind you should upgrade too, but these come when most users are generally experienced with using their character.
If you have any further questions, I recommend consulting the
Wiki. Most commonly asked questions are generally answered there, but feel free to post additional questions if you have them.