Q. Which is better – to love God or to fear Him?
A. On the Sh’ma passage that appears in the Torah (Deut. 6:4), Rashi says, “’Love the Lord your God’ – perform His commandments out of love, since a person who serves Him out of love is not to be compared to one who serves Him out of fear”.
When you serve God out of fear, your heart isn’t in it. You don’t act in a certain way because you desire to but because you want to avoid punishment. When the element of punishment is not present you may find yourself breathing a sigh of relief and immediately abandoning the mitzvah.
On the other hand, if you serve God out of love you are ecstatic at the thought that you have a way to show your love. Instead of looking for ways to avoid the mitzvot, you happily and joyously welcome every opportunity of performing acts of love. Possibly you will get a reward, possibly you won’t. The reward is not the main thing.
As a postscript, let me add that fear has two dimensions – one connotation is to be afraid, and that seems to be what Rashi is talking about, as well as the approach that I have taken in this answer; the second interpretation is “reverence”, having a feeling of awe. It is in that sense that the Amidah refers to God as ha-gadol ha-gibbor veha-nora – “the great, mighty and awesome God”.