Does it depend on how many small items you're carrying around vs. larger items? For the sake of sanity, let's assume it's not hardcoe so ammo doesn't affect this. I'm thinking that by level 8 you'll have 2-3 weapons you use often and still be collecting high weight crafting components (Eg. Fission Battery and Steam Gayge Assembly from FO3).
If this is the case, then Strong Back might be the better choice. Later on once you're near the max level you should also rely less on 1 weight items and carry more situational weapons (I know I did in FO3), so Strong Back might win there too.
I bet there's something obvious I've missed in this decision though.