A big question is: what else do you want to use your LRMs for? If the answer is, nothing, then just take an LB5 or 2 or something like that (though I do favor the lawn dart check, so if you're not using that the LB, despite it's incredible TN bonus, may be a worse choice).
But, the lovely thing about LRMs is you can also shoot mechs, and tanks (LBs are good for that too) and BA, and all that. As you point out, you can use indirect fire. So, a quick bit of math shows that three LRM5s offers you more ammo, meaning more special ammo options. Lots of tiny mine fields? A semi-guided option? NARC? Sky's the limit. The LRM15 is better if you're thinking of using an Artemis system (some like them, some don't), and you get more 5 point hits which is good for many reasons. And if you just need damage, you can do four LRM5s, which hurts your ammo (12 salvos) but more hits and more total chances to hit (vs all or nothing with the LRM15).
I tend to favor bigger racks just for ease of play, but it's pretty nearly a toss up I think.