A balance of forces is the way to go, like
beanerboy has suggested though I would take it to another level of consideration.
The points value must be considered against the actual and relative ability of the mini, whether it be a humble hobbit, an angry uruk, or the Scorekeeper-in-Chief, Master Gimli. Actual ability is what the stats for any mini say they can do. Relative ability is how any given mini measures up against one or more opponents, so for instance, you could reasonably expect Gimli to beat an individual goblin, but he may not fare so well against 12 of them, or a Mordor Troll... you get the picture?
If your opponent has piled everything into a mounted force, they may have fewer minis but they have far greater manouvrability, and may be able to exploit that to bring more favourable odds to bear in each actual fight, picking off your units one by one....
So don't think solely of how many points or Mights or whatever you can get for any given overall points limit - think about how you can best use the forces available. Play to your strengths and you opponents weaknesses.
But one or two bad rolls of the dice and even the most capable army with the most cunning plan can come badly unstuck...