As the title says.
Each faction generally has some very good units, but they tend to either cost more or cost less pop points so it will cost more to fill out a fleet.
An example is the Nebulon B for the New Republic/Zsinj. For the pop points 2 of them are better than an quasar fire but for a 1 point unit it costs a lot of money, around 12k for 6 of them vs 6k (or less) for a capital ship costing 6 points.
Another example is the strike cruiser, a 3 point unit that often costs around 1200 credits vs a Victory class that is a lot more expensive.
For me I usually go with quantity, at least early on and then switching over to quality later on once I have my economy up and running. Cheap fleets often get used on the defensive or to fleesh out a fleet pop points. A good example of this is Zsinj with the Lucrehulks or filling out a NR fleet with cheap mon cal ships+ the quasar fire. Sure I would like a heap of marauder corvettes and Nebulon Bs but they cost money. For Zsinj 3 or 4 ISD IIs+ a pile of Lucrehulks is a cheap fleet and on the defensive a pile of Lucrehulks+Quasar fires+ HVG can be tough for the AI to crack.
I will go for quality over quantity is if the price is close say a Tector vs an ISD II where the extra 33% (roughly) doesn't matter so much.