I'm not a fan of GA since it got nerfed hard unless you have a ton of HP backing it up, which you won't if it's your only defensive item. Still has its uses, though. Warmog's has been nerfed so many times I don't imagine many scenarios where I'm buying it over Frozen Mallet anymore.
Depending on team comp, you may not even need a defensive item. Tristana in particular is the safest lategame carry innately, having both a jump and a targeted (and AoE) knockback along with the largest range.
Typically you get it after you get 3 damage items.
Like on trist, I'll go BotRK -> IE -> LW -> Def item -> PD
Sometimes you can delay it, sometimes you might want to buy some of it early (Like a giants belt after part of IE or something).
If enemy team has AD assassin -> Randuins > Warmogs > Frozen Mallet > GA If enemy team has burst AP -> Warmogs >Frozen Mallet > Banshees > GA If enemy team is poke/sustained(AP) GA > Warmogs > Frozen Mallet > Banshees
What do you do when faced with two choices? Simply toss a coin. It works because in that brief time that the coin is in the air you know what you're hoping for.
You should not be looking to buy a defensive item until after your LW (in most cases). What you buy is situational, look at what champions will/are diving you every fight and build to survive them. For example, if you have a zed diving you every fight, warmogs and randuins are both good options. All of the defensive items are situationally useful, and outside of thornmail, you can probably find a case where one would be better than another, so think your purchase through a bit. Sometimes, the best choice will be to skip any defenses whatsoever, this is especially the case against heavy sustained damage teams (champions such as singed, ryze, karthus, etc.) - where your best choice may be a second lifesteal item.