Thancred is a mixed bag, but can be quite useful once Dual Delay is available, though he can't use 4* Support. His Defense is very low, making him a Glass Canon, like Lightning. However, he has far better equipment choices than her, especially in regards to Armors (he can equip almost everything, including Helms, Light armor, Heavy Armor and Shields). If you get a good Bow (or a 5* Instrument), he can do a fine job on the back row.
Y'shtola is a very solid White mage (who can equip Light Armors and Shields - while Shields are often considered so-so, for good reasons, even those are useful with synergy; AFAIK, she is the only dedicated White mage able to equip shields, aside from Tyro). She has access to 4* Support, allowing her to use Breakdown abilities if you need to, though you will equip her with Protectga, Shellga or Diaga more often than not. That being said, those are very nice perks.
On the other hand, compared to other dedicated White mages or even some Summoners (like Eiko), her regular Soul Break is useless. Thus, she only really shines if you manage to get your hands on her Soul Break relics.
