Like most Magic: The Gathering cards, the value of Summon: Bahamut depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Final Fantasy)
But I can imagine there are shells where Sephiroth performs even better
Celes was considered for Final Fantasy 6, as the set revolves around the World of Ruin, of which she is the clear focus rather than Terra, while Yuna was also considered for Final Fantasy 10
"Iago - Giant Spectral Parrot" is the other featured card, and while his initial stats seem great - he can be removed from play easily if you aren't careful
Each card has either one or a combination of five colors
In the end, we decided they could coexist as separate mechanics that are cool in their own ways