For priests, why is Conjure Fire Elemental a level 6 spell and Conjure Earth Elemental a level 7 spell? The spell descriptions are exactly the same.

It seems to me that Fire elementals are more useful because they are immune to fire. What's so great about the Earth elemental that makes it worthy of being a higher level spell?

IIRC Earth Elementals have slightly more HP and do more dammage (crushing) these make it slightly better but it is still a lvl 7 spell. This makes the Fire elemental more popular as there are not many good lvl 6 druid spells. There are however a few excellent lvl 7 spells (natures beauty etc). So most people make their preists memorise Summon Fire Elementals for the 6th lvl spells and not bother about the 7th lvl Earth Elementals

Afaik there are few to no creatures which are resistant to bludgeoning damage (and I am not talking of stoneskin here). Otoh lots of creatures are resistant to slashing and/or fire damage, which I beleive the Fire Elemental does.
Does anybody remember the enchantment of the fists of the two elementals?

No, I don't know the enchantment. I think the Fire Elemental's damage is physical. There might be added fire damage, like melf minute meteors, but I can't seem to remember.

So the biggest difference it seems is the type of damage - blunt versus slashing.

Does anybody remember the enchantment of the fists of the two elementals?+4 or better. Fire Elementals can hit Kangaax, iirc. I haven't tested out Earth Elementals much, so I'm not very sure if their fists are high enchanted. Shrikant is right that very few monsters in the game have resistance to blunt damage, though. It's why monks are so popular.

I do not think there is a that much big difference, as mage versions are the same level.
Even I like earth better, I would prefer fire anyday as it is more useful. You say earth does bludgeoning which few monsters resist, but fire is still more versatile, IMHO, heck, they can even hurt Kangaxx, this is NOT a small accomplishment.

Druids should not waste their PRECIOUS Lvl 7 slot with Earth Elemental. (or until level 15, where they get absurdly high spell slots!)(though they need an absurdly high xp too!)