This might be a bit of a ramble, but hopefully you can salvage something from this.
SMG's increase the amount of shots you have overall in your squad, so more dice for you during your shooting!
Here is an example of my Polish army and it's weapon options:
1 NCO and 9 Regular infantry = 100pts
1 NCO and 9 regular infantry + 1 bar = 105pts
1 NCO and 9 inexperienced infantry = 70pts
1 NCO and 9 inexperienced infantry + bar = 75pts
Weaker units, but less points spent for more rolls. If I wanted to add more units I could, but obviously I only get to add one bar.
So what the heck am I even on about?
Regardless of whether your troops are inexperienced, regular, or veteran, you can compensate for lower numbers by adding an automatic weapon to increase the amount of dice rolled. Obviously you want more models for inexperienced groups, but you can see what I mean when it comes to elite groups
Hopefully that helps. Veteran troops are awesome, but make sure they can hit as hard as possible! Because of that, I would say field as many SMG's as you can, simply because you can use cheaper infantry groups to take the main fire while your smaller groups can flank the enemy with some serious damage. Of course, these upgrades aren't exactly expensive, so you can always add upgrades to your cannon fodder too. It really is up to you.
it will all depend on your play-style of course.
Sorry if that makes no sense, but maybe that helps