Ask yourself: if that guy will get in my face, will he melt it?
Does the enemy team have some physical damage dealer that can easily get to me and pop me like a baloon?
Does the enemy team has yi/someone which is slow resistant?
Is the enemy full of squishies?
Am i ballsy enough?
Am i fed?
If the enemy has like nidalee/leblanc or similar go triforce, if the enemy has something like illaoi/kha/darius/rengar go iceborn, but this besides the matchup has a lot to do on how you want to play him, i personally would always go trinity, but that's because i'm a greedy fuck and i value splitpush more than fighting also.
If the enemy has olaf/yi iceborn would do nothing against them, in the contrary, if the enemy has like talon or kha, can you slow them after they used their gap closers? will they just oneshot you as soon as they touch you without giving you time to E away? How good is the peel from your team? is that a poke comp?
Because you can either choose to kill kha from range before he gets to you or prevent him to reach you by slowing him.
Just consider all that and see what works best for you.
Also remember that once manamune is completed it will stack on autos too, so it will stack faster, you should have muramana by 20-25 minutes, and i personally suggest to go 6 blues of flat cdr and 3 of scaling cdr, so you will be able to go berserkers over ionian, which i prefer.