Here's how I've been taught to think of it when trying to choose.
Relic Shield: Melee champs only can activate the charges (works on Thresh because he's half melee). This helps you get up in the enemies' faces during laning, take some poke, get a heal for yourself and your carry, and still earn some gold for your effort. Oh and that late game shield is great for your divers and getting your ADC out of the pickles they keep finding themselves in.
Frostqueen's Blade: Take this if you think you can poke enemy champs with your AP spells often (which is why it's good on Sona). This one is riskier because if you take it but can't get the pokes in, you're loosing money.
Talisman: Take this if you think you can't poke or if you're playing with a team that specifically needs the speed boost (taking it on Sona when you can't poke is actually hilarious because, you know, speed up again when one speed boost runs out). This is a "safer" option because you will gain gold just for being near minions that die (not necessarily on the front lines risking your face to that insane Cait range), but I think it is a little lower rate than if you have a poke heavy composition that lets you make use of Frostqueens instead.
In essence it comes down what you expect the game to look like. A good support can gauge what kind of lane they are going to have (minus the actual skill cap of the enemy duo or the knowledge of your ADC - it can sometimes be hard).
For instance, my personal style is to ALWAYS take Frostqueen on Nami, because I will gladly just poke/heal myself with W and make my ADC scream "Retreat you retard! Oh wait, free kill. Gotcha. Now please save some healz for me next time, kthx."
But Sona is a little harder for me. I find I have a hard time poking without taking massive autos the entire time. It may be because I'm too aggressive, it may be that my ADC isn't following the poke up when they can. It may be that I just underestimate her in-built heal. So if we are going against a crazy poke heavy comp, I will take Talisman, or if I know my ADC will help me poke when they can, I will take Frostqueens. It really just depends.
Hope this helps! I'm actually gonna post this as a guide.