Up to a point, you'll get a bit more bang for your buck by concentrating your investment, but EXP isn't the resource to be concerned with. EXP scaling takes care of itself. Low-level characters catch up fast, and being in a well-designed unit makes a much bigger difference than raw numbers, especially once you gain access to highly synergistic abilities that blow the ceiling off a unit's actual effectiveness. Rather, what you want to be thinking about is honor.
There are five things to spend honor on.
- Hiring mercenaries
- Unlocking units
- Expanding units
- Promoting characters
- Forging weapons
Hiring mercenaries is only something you should pursue if you have a specific unit-building goal in mind that you can't fulfill with unique units. You'll get at least one unique member of every recruitable class, and even if you skip a bunch of optional uniques you'll end up with more than you can deploy. The advantage of mercenaries is that you can access them earlier or in greater quantity, in order to pursue specific team-building ideas. If you don't know whether you want that, you don't want it. Mercenaries' levels scale based on the location of the fort where you hire them, so there's no risk in delaying.
Forging weapons is a late-game honor sink that pretty much only becomes relevant once you've maxed out the other things to spend honor on, so don't worry about it. (Though, once you reach the blacksmith, you should consider forging the Kingsblade Cornix.)
So that leaves unlocking units, expanding units, and promoting characters as your highest priorities. You should be doing all three, as neglecting any one will leave you hard pressed in extreme cases, but some of them will be more impactful than others.
Unlocking units has severely diminishing returns. Having a large number of units gives you more strategic flexibility in the large story missions, but only if you've got enough characters to fill them out, and only the biggest missions will have enough going on to need that many units deployed at once.
Both expanding units and promoting characters add a minimum 1 AP and 1 PP to a unit's total, drastically increasing their power, but there's nuance to choosing between them. You might get more than 1 AP and 1 PP if you use a promoted character's extra accessory slot to wear a pendant, or by putting an already-promoted unit into the new character slot. A bigger unit can spend its AP and PP faster, performing more moves in each turn, but a unit with promoted characters can spend them on better moves that can combine more impactfully.
Between the two, I usually found that a promotion made a bigger difference than an expansion, since you could usually set up a nice lean combo that's sufficient to keep common enemies at bay, but it was important to have some elite units that were larger, which could deal with major threats and operate without backup. Additionally, it is pointless to go to the maximum size of 5 too soon, since such units must have at least 2 frontliners, and not every character can hang in the front row without the right equipment.
I wanted to use as many characters as possible, so I made sure to have enough slots for every unique character, but others have reported that concentrating their investment on having fewer but stronger units worked well for them.
