In many cases, the level of a spell affects the duration (the higher your level, the longer it lasts). For example, with one of my current characters, the duration of detect invis was 24 ticks in logs of me at lower levels, and 56 ticks at hero.

Level also directly affects how difficult it is to dispel. Level 51 detect invis is much harder to dispel than level 20 detect invis. That's probably not very important in the case of detect invis, but in the case of your fly preps, for example, it's something to consider.