I've got an impressive fleet, but I just have not played enough to really master it and play competitively in a tournament setting. But I do have some questions that maybe y'all can help with.

If you had a choice between getting a target lock or getting a focus, which is better under what circumstances? I have not crunched the numbers but I'm tending to believe taking the target lock is more important because it can carry over to the next turn, but it cannot be used defensively, and I'm not so sure that at if I could only get a primary weapon snapshot off the target lock is more desirable than the focus (plus the focus has some defensive value).

Another action dilemma that follows on is that if I have a primary weapon shot and only a target lock (but no focus because of, say, stress), do I take the shot and cash in the lock on a mediocre or bad roll, or do I just shoot, keep the lock and unstress the next turn hoping I can focus and get off that missile or torp?

(Take this all with an "in my opinion" attached.)

If you had a choice between getting a target lock or getting a focus, which is better under what circumstances? I have not crunched the numbers but I'm tending to believe taking the target lock is more important because it can carry over to the next turn, but it cannot be used defensively, and I'm not so sure that at if I could only get a primary weapon snapshot off the target lock is more desirable than the focus (plus the focus has some defensive value).

As you say, target lock has the advantage of carrying over, as well as generating crits at a slightly higher rate, but the disadvantage of having no defensive use. Focus has the flexibility of using it on either offense or defense, but is wiped away at the end of the round.

Personally, I prefer focus for low-PS pilots, because they'll often be attacked several times before they can attack themselves, and focus helps make sure they get an attack. I also prefer focus for any pilot who will likely take concentrated fire from the opponent, for similar reasons. I typically use target lock for high-PS pilots in relatively low-risk situations, and for ships with 1 Agility, where focus is just less useful for defense.

Another action dilemma that follows on is that if I have a primary weapon shot and only a target lock (but no focus because of, say, stress), do I take the shot and cash in the lock on a mediocre or bad roll, or do I just shoot, keep the lock and unstress the next turn hoping I can focus and get off that missile or torp?

Remember that damage now is almost always worth more than damage later; take the shot, and then look at your dice and your opponent. Against, e.g., a TIE Fighter with focus, if you roll 0-1 hits on your naked dice you're unlikely to get anywhere on that roll, and you want to hold on to the TL. Against a stressed B-wing with no tokens, it might still be worth spending your TL even if you're rerolling all of your dice, because that shot still stands a good chance of getting through. If you roll 2 hits and a focus, you might reroll that last die against the TIE Fighter (because you might need 3 hits/crits to get through), but against the B-wing you'll definitely hit and you might want to hold the TL as a way to salvage next round's attack roll.

Depends on who you are using. Jendon and Vessery make taking target locks ideal.

Focus is great, but target lock can solve focus and blanks.

For focus, the more dice you are rolling, the more effective focus is. Also focus doubles as a defensive action.

Without focus you have a 50% chance of rolling a hit. With it you have a 75% chance.

Target gives you ~75% chance of hitting with each dice. So from this perspective it is worse because it gives you similar chances to hit, but no defensive bonus.

BUT! Target lock persists between turns! So if you don't need it, it helps your next turn too, not just the current turn!

So this is my rule:

If I'm at risk of being fired at, I typically focus, because it has the chance to help me if I get shot at.

If I'm not at risk of being fired at (or just not worried about getting hurt) I target lock, because if I don't need the target lock it will help me out next turn and combine with focus.

So this is my rule:

If I'm at risk of being fired at, I typically focus, because it has the chance to help me if I get shot at.

If I'm not at risk of being fired at (or just not worried about getting hurt) I target lock, because if I don't need the target lock it will help me out next turn and combine with focus.

a great rule to follow

The value of both tends to vary with the number of dice you can modify. I usually look at it defensively first, so if you don't have a chance at getting shot, you can skip right ahead to whatever maximizes your offense (target lock slightly better). Saving a focus for defense on a 1 AG ship rarely provides much value unless you are desperately close to getting shot down, so I'll generally target lock when I have the option. On a 3 AG ship, there is a much higher chance of rolling a 1 or more eyeballs for defense and the drop-off for offense is minimal, so I'll generally focus. On 2 AG, it's pretty much a toss up that's solely situational and whatever's your preference, but generally the flexibility of a focus tends to be the best default choice if you can't decide. Regardless, count on rolling eyeballs when you Target Lock and rolling blanks when you focus. The dice know your decision long before you ever roll them.

You can also use Target-Lock to make your ships a more appealing target to draw fire away from your already damaged ships, who focus.

If you plan to overshoot or be out of arc on your target, take a TL. If you are going to stop short in their arc at range 1, take a focus.