Well, I want to say Ifrit because he's simply a higher level summon in III (and possibly V?)...
On the other hand... when you fight Ifrit in VIII, you can tell he realizes he's screwed when you use Shiva.

I can definitely understand the logic about coming to a tie... Ifrit only wins that easily if they're Pokémon.
blaugh I might say Shiva because she seems like she'd be able to get more hits in and dodge more easily. Of course, in most games, you get nothing but their attack, but... In X, anyway, Shiva's evading was a huge help. As such, I'll go with her.