The difference is twofold: narrative and mechanical
In the narrative sense, the PCs are the protagonists of the story. They make the decisions and drive the action. A player-controller NPC, in contrast, is usually a secondary character, an extra, who tends to mainly follow the PCs into action than spark action themselves. A typical example is an animal companion of a ranger or a familiar of a spellcaster - they are clearly separate characters from their master, but still subservient to the main PC. They're there to help more than act on their own.
Mechanically, the NPCs can be built much lighter than the PCs, depending on their intended use. For example, if you want to give your players some combat support, you can essentially just pick an appropriate stat block from the Monster Manual and use it as the NPC - this is how I have done it. The resulting character has all the stats it needs for a single combat, but no personality (bonds, flaws and the like), background or hit dice.
However, you can also make a mechanically complete NPC with the normal character generation rules. This is often a good idea if the NPC will remain with the party for a while, as PCs are balanced for extended adventuring days while monster statblocks are balanced for single encounters.
Which do you want?
There are many issues to consider, but I think the most critical one is complexity. If your players can manage and remember the features of two different characters in two different classes, including both the mechanical and narrative aspects, then creating two new full-blown PCs is a valid option. If possible, I recommend it, because having two characters of hopefully different classes is sure to enrich the ways your players approach their problems with.
If you feel it would be too much for your players, make them NPCs: either build new PCs using the normal rules but simplify their characters, or go the full way and make them minimalist monster stat blocks. This is a good option particularly if you find your party needs "fire support", but as your issue seems to be more with the lack of variety than power, I recommend full PCs more.