HP ice isn't SUPERIOR to fire. It just does a different job. I don't like being walled by Scizor/Ferrothorn so I use HP fire. Plus my team doesn't have an exact counter to Ferro, so thats another reason why he has it. Absorbs seed and rips it to shreds with HP Fire. Not so effective in rain though...
i told you without sp.attack investement amoongus cannot 3hko specially defensive nattorei,which is the most common ev spread i see...after nattorei sees that you have hp fire he is just going to switch having only lost 35%(more or less).
0 sp.attack amoonguss vs max hp/max sp def using hp fire:119-140.
after 2 turns of leftovers it misses out the 3hko by 2 hp...of course nattorei almost never will be to exactly 100% so most of the times it's a solid 3hko!
also scizor almost always u-turns on you so you don't get to kill him with hp fire,though you could always hit him in the switch but this is not the most easy thing to do...
anyway what i am saying is that i want my amoonguss to avoid being setup bait...i already have spore to deal with nattorei which is more than enough...if the opponent has a nattorei and amoonguss comes in,nattorei is almost surely the one that's coming to wall him so you get to spore him...hp fire also is shit if drizzle is on so that's a downfall also!
in the other hand hp ice hits many things that hp fire cannot and it's very useful in my opinion...morobareru can check things like salamence,garchomp and dragonite in a pinch since nothing they carry can ohko him if running some defense evs even at +1,hitting them for huge damage with hp ic.eg.morobareru with 206 sp.atc using hp ice against:
-min hp/min sp.def salamence=217-255(100% ohko after sr and life orb damage)
-min sp.def garchomp=207-243(100% 2hko after sr)
-min sp.def dragonite=181-213(most of the times 2hko,and also most of the times breaks sub after dragonite uses roost)
-min sp.def/min hp voltolos=108-128(always breaks subs and 2hkoes more than 85% of the time after sr)
in most of these situations adding a little bit of sp.attack evs helps tremendously with the above threats while in nattoreis case you will always 3hko him no matter how much sp.attck evs you run...(of course i am talking about logical ev investemnent not 252 evs in sp.atc).
i prefer to be able not to give to voltolos or to garchomp a free chance to sub in my face than to 3hko nattorei which i can already cripple with spore.
to end my babbling i think that all in all hp ice is the most reliable and stantard option(in terms of usefulness)and hp fire handles special things and is for more dedicated roles(for example if your only way to reliably kill nattorei is amoonguss then of course you will use it)...