I have mained Frozen Orb pretty much since the game's release and probably have a bit more experience than some here.
Over Death's Fathom, Doom is approximately 9-10% higher dps against immunes (also depends on whether your sunder is -res or %cd, can be slightly more or less depending), while it suffers considerably against non-immunes. Depending on skill distribution and gear, Fathom deals up to almost 55% more dps against non-immune creatures, although admittedly in testing I had mistakenly put my Frost jewel on my Fathom, so I was losing that as well. Edit: I did at least have a normal facet in my helm, but there was definitely user error in my most recent testing.
You need to ask yourself: what is more important to you- dealing slightly more damage to immunes, or significantly more damage to non-immunes? Are you only fighting cold immune heralds, or are you bossing and doing general content?
The above makes Fathom significantly better in my opinion, although it is arguably much harder to get, especially a high rolled one. There are also some things people just neglect to bring into this conversation, such as losing access to fcr on your weapon by using doom. The Fire Resist on Fathom isn't something that should be dismissed either, even if resists are a bit less important on ES.
the purpose of doom is that blizz/orb >> pure blizz & pure orb. holding doom allows you to dump points into orb rather than cold mastery so you can use it between blizzs on cooldown