Rain is a better fullbreaker unit. Cloud, I would save for a better banner. The banner is not bad by any means, but I don't think it has anything for your gaps
Yeah I would agree with you that Rain is the best breaker right now.
But we can't overlook Ling's utility in a party. She offers a lot that it's really hard to overlook her, specially if you're new. Let's see what she can offer in a party: AoE Raise, AoE MP Recovery, AoE All Status Recovery, LB Rate Up, Entrust. Only downside is Ling's squishy as a jelly.
For a beginner, she's a godsend and too valuable to overlook. For veterans, maybe not as much since we have dedicated units that do a better job at a specific role.
What the fullbreakers do?
Maybe I have a unit for that
Breakers are units that inflicts status debuff on monsters, reducing their damage, defense, magic damage, magic defense, elemental resists etc. It makes it much easier to beat some trials that does not resist break skills.
My personal opinion, Ling will be a great addition to your party. But She's a 4* base, so it's harder to pull her than a 3*. At 30 tickets, their's a real high chance you won't get her. So think it over.
Alternatively, you can wait for FF12 banner, which has Tidus and Rikku. Both serves the same role as Yun and Ling, which is dps and support respectively, but better. But they won't be here for at least 3 months.