I vote style but I might be lying to myself. I play BA because assault is the best part. First army was Tau and I just hated sitting back hoping no one would attack my guys. I decided I wanted to use assault style marines (style) so I made Vanguard Vets because they are better than Assault marines (substance). I then Math Hammered out what the best weapon loadout was by something I can only call nuking it out (for those not in the US Nuclear Navy it means trying way too hard to figure something out that isn't that hard) which again implies substance. I am always going to use death company (I play Blood Angels... style) and gave them Lemartes (substance). I wanted to use Sanguinary Guard (style) and I plan to pack the squad with things that will buff them out like crazy (substance) but will keep the swords and not power fists because the swords look good (style). 10 scout snipers because I wanted to try painting Camo (style). 20 Tacticals in a Rhino because I wanted to see if I could do magnets on my predators (style or trying too hard). 10 Primaris because why not?
Basically all squads were chosen due to style but then I nuked it out to see what makes what I like the best. (P.S. Vanguard Veterans should be given a chaplain, sanguinary priest, and two chainsowrds each because my nuke math says so!)
I think most of us do this. You pick what you like then make sure it's as good as possible. Paint what you love then make sure it's able to do well in the game.