Alaric Drusus:
Tactics is my next choice. It's fair to say that, being able to move after the other guy is likely more advantageous than having to move before him, but getting to make yourself hard to hit. Tactics also helps with increasing NCO ranks down the road.
Teri makes a good point.
You should really consider what youre piloting and how much youre going to want to modify it.
Lucius is piloting a heavily modified Wasp. (Message 4)
I'm piloting a heavily modified Locust. (Message 5)
They're not going to be brute forcing through any missions but they're both a lot leaner and meaner than their size might suggest.
link to a message in another game
You're not going to be able to get where we are out of the box, but if you dont think about it, and start gathering what you need you'll never get there. About the only nice thing about 20 tonners is they're inexpensive to modify.
Keep in mind the modification caps though. base 2 tons of modification for a 20 ton mech, plus mental ability ranks. Though if youre in a Stinger-3G, really there isnt a lot more to look at to trick it out. I think your biggest issue is that you dont have enough guns on it. You have two medium lasers on it, and Joaquin will be the first to tell you, that on a mission, two medium lasers feels under gunned.
The surest path to survival is to bury the op force before he buries you.