I doubt that Stonewall will answer you so I will.
Trundle is total niche. There is no reason to pick him in SQ. However in Team Queue he can fit very well to many compositions. He just need teamwork to be good.
Sejuani now is better pick, but not the best. She has AoE Ranged 2sec stun which is nice engage/counter engage tool, but besides she's not good overall. Slow early clearing, moderate pre6 ganks, moderate early/mid game dmg, very good lategame. She can fit very well to high dmg low cc comps.
Which one is best pick to win? Jarvan IV. Xin Zhao. Vi. They have hard cc too, slows, much higher damage potential and armor penetration. In particular Jarvan - his ult is just op, free stats, %hit damage, 1,5 sec knockup every 10 sec or so and awesome damage. He has nearly same tools as Sejuani, but better. So in my opinion to win is better to pick Jarvan over Sejuani, but she's not bad and can win too. Some comps are better for Sejuani than for Jarvan, she can fit everywhere where damage is.Trundle is too team relevant and SQ is place when you're punished for teamplay. He's not good pick on SQ.