So my two favorite champions/mains are two off meta ones lol

depends, I personally favor IBG over tri but I also just don't like tri very much, If you are against an AD heavy team, You are going very tanky and building a lot of armour (makes a bigger slow field), or your team has plenty of dmg and you don't feel like you need to build dmg. In any of these cases I would say that IBG is better. If you do need to help with dmg or they don't have a ton of AD it is up to you. Tri gives more speed vs mobile enemies which is nice but if they have things like kog, varus, syndra, or ziggs then it is totally up to you. The slow hurts them a lot if you can't burst them/don't have the backup, the movespeed helps if you are going to be jumping into their team/trying to get them to your team. There are a bunch of variables that you have to keep in mind but they are both strong enough with skarner that you shouldn't be overly worried about making the wrong choice. Also Tri can be better for sieges because it allows you to snag someone that gets too close.