Alright, I'm really wierd...
I combine everything, really.
First or so time through:
I often use characters that are good upon joining, especially if they are Percival or Seth epic upon joining. I also randomly nab units that fit my play style, or just random units I think will be fun to use. In addition, if a certain character gets my attention for personality or appearance (lol Sigrun [<3], Lucius, Soren, Ilyana, Lowen, blah blah blah) I'll use them.
So I guess I need to mention my playstyle type things:
I tend to favor using lots of units that kill things fast. I like doing things fast. Maybe I just favor good units or something, but I like success. :D
On subsequent playthroughs I tend to try to use most of the units, and then playthrough with lots of good units, lots of bad ones, themes, whatever.