My experience so far
Level 5 - 759/950 (574 GDI - Sector 3, 185 NOD - Sector 1)
This is all going to be focused more on GDI since I haven't played NOD that much yet (playing to farm coins for training those units).
Riflemen - Max LV4, Missile Squad - Max LV5, Rhino - LV3 no training, Pitbull - LV4 no training, Predator Tank - LV5 no training, Wolverine - LV4 no training, Zone Trooper - LV5 no training, Sandstorm - LV5 no training.
Army Setup - Riflemen, Missile Squad, Rhino, Pitbull, Predator Tank, Zone Trooper, Dr. Liang
Riflemen appear to be on par with each other in an encounter so long as they arrive on opposing tiles at the same time. If not, it's usually the approaching unit that gets shredded in the end unless sent in pairs or with the Rhino.
Missile Units - oh, the Missile Units. Jesus christ these guys. With the exception of infantry-based anti-infantry (riflemen and I'll assume sniper teams since I haven't encountered them yet), these guys are a hard stop to any vehicle. Rhino? Missile Unit takes it out. Pitbull? Missile Unit takes it out. Rhino? Missile Unit takes it out. Talon? Missile Unit takes it out.
Multiple Talons? Just add one more Missile Unit for every two Talons. Wolverine? Two Missile Units. Titan? Two Missile Units. No idea on how they fair against the Mammoth Tank or Disruptor though. That being said, you can save resources using missile units over using a predator or pitbull.
Pitbull - Great for taking out Rhinos and harvesters, but about the best I've found for it.
Rhino - Great for Riflemen, but that's about it. Dunno how they fair against air units like Talons and such, since its easier to just use missile units imo.
Predator Tank - Great for Rhinos and Pitbulls, as well as stalling tech lab units.
Tech lab - rarely bother using or have need of using. Sandstorm's 50% damage per additional target is hell against someone trying to defend/push back a point with multiple units.