Quantity or Quality


Quantity is almost always better. Upgrading a tile costs the same as just making a new tile but only gives like a 20% boost. However, if you make a new tile, that effectively gives you a 100% boost as you now have double the ants you had before.
There are exceptions though, for example fire ants and rapid fire ants getting extra abilities or extra units when you fully upgrade them, or little black ants as they're already very cheap and weak at level one.
Once you get a certain number of ants though, it's generally better to upgrade them, as you can only fit so many ants around an enemy at a time, and your frames will start to suffer if you make too many.