(Definitely-not-Dave).
.50s. Just put .50s everywhere.
Maybe ;b
On a more serious note it does depend on what faction you're playing either OST or OKW.
By the point you're in mid game is where I considered USF is at it's strongest as your armor is equal to Axis medium tanks more or less and Axis hasn't had the opportunity to make a late game defense.
If playing on a map where certain buildings dominate the entire map flow or urban heavy I'd recommend Rifle or Armor company for flamers and use the USF mortar to smoke MGs out of buildings and use to smoke to force OST relocate or use the USF mortar just like any other mortar against OKW. If either of those companies don't suit you then the mortar half track hard counters OST mortars whilst also being incredibly strong against infantry squads. Not to mention the WP rounds can actually do more damage than a normal barrage when a MG is in a building.
Airborne I find actually more useful for CPT as you only have to wait till 2 CP to .50cals where LT has to wait till 4 CP. .50cals sorely provide that ability to lock down sectors, help reduce bleed, and assist in assaults due to it's quick set up time. Paratroopers also find more use in CPT tier than LT due to CPT while still being good at AI with a BAR isn't as good as LT due to his veterancy. LMG or Thompsons work well either way depending on map, your micro, and ability to combine abilities (i.e. CPT using On Me! on Thompson paratroopers) Path finders are also generally a manpower sink as they don't suit that long range combat when you need to be mobile, aggressive, and close to maximize damage with USF (mostly). Now if you do float manpower they can be decent due their great line of sight. Might actually use them more and get back to you.
Now shockingly if you're far enough ahead or have the chance to place munition or fuel caches, going grenades and weapon upgrades can really catch people off guard. No USF players really go grenades anymore till late game and even then half the time they still don't go it. Using smoke and BARs to assault a position lets you not only dominate OST in close range, the grenades can give you that edge against OKW.
Anyways, hope this helps and good luck.