Mass Effect

I believe that you do get more experience on hardcore and insanity but I'm not sure because my characters level has been maxed for my last three playthroughs. A suggestion though, the difficulty of your enemies relates to the game difficulty (i.e. normal,hardcore, etc.) not to your characters level, so level your character up on an easier difficulty and beat the game then start a new game on the higher difficulties using your already leveled up character. Also, you may get more advanced equipment earlier on higher difficulties.

hardcore difficulty, enemies scale to your level, and have immunity, insanity difficulty, enemies are slightly higher then your level and have better immunity. Harder difficulty levels are just for a tougher experiance, and achievements, if you don't like them, just finish the game as fast as possible to get the achievement on those two levels, then play on whatever difficulty you want.

The scaling of the enemies to your level accounts for the higher amounts of experience you gain. The base experience which mobs give is the same between difficulties.

And trust me, the game is a lot more interesting on Hardcore and Insanity difficulties. The game is way too easy on Normal or Veteran; it really doesn't make you play out strategies with your powers and teammates like Insanity does.