Playing as the union I guess?
The veteran slider just effects the overall of the replacements, not the unit itself. Once you get a veteraned unit with chevrons/stars it starts to matter as you can lose the ranks (max 3) and the benefit. Those rank bonuses can matter. Keeping the brigade commander alive matters more imo, as unit respond slower if he is dead/wounded.
Have you been spending your government points, not sure the actual name? Top right corner in the camp. You can buy more troops that way.
I think the benefits on your rifles is minimal and wont make or break a battle. Cannon choice matters of whole heck of a lot more imo. Parrot guns are the only thing worth bringing that I can get in numbers. And then not all the time. For skirmishes it's also nice having longer range rifles.
But for the line infantry positioning and cover are the biggest "multipliers". Some like the Henry and Spencer are worth it for the reload. But really the only difference is range. And the only way to tell is have guys in the exact same conditions/training/skills.