Grudging gamma Eptagon objects to the straightforward math proving Beastly Appendage's value at XL5. Read the context there; I'll only quote the portion to which I'm responding.

level 2
Eptagon
1 point
7 hours ago
Keep in mind the user who provided the info has never won a game, despite aggressively preaching to some of the highest level players and being so confident in their opinion. Make of that what you will.
For the matter at hand, specifically, consensus leans towards the helmet because it's the safer choice. You can nope out of a fight if things start to go South, while fsim assumes you just keep mindlessly hitting the thing.
I don't have much experience with MfTm, as most of my transmuters have been octopodes, but I tend to swap to Ice Form as soon as possible, which makes BA pretty much useless. In your situation, either could be the right choice.
As /u/wazzur1 mentions, both Ice and Spider Form have some disadvantages, at least for a character that can actually wear armor, so BA might be the correct choice (killing things faster is, in fact, a good defense) if you don't find yourself taking damage too quickly.
If you're consistently using a form or get Blade Hands online, the helmet is unquestionably superior to having BA available.
wazzur1
1 point
5 hours ago
It's easy to underestimate Aux attacks, but they are very strong. I play lots of Ds and know from experience how much your DPS goes up in early D when you get a combat body slot mutation.
The question presented here is essentially 1 AC vs ~4 slaying or so. There are other considerations at play, but in general BA should be kept until blade hands are online.
level 3
Eptagon
1 point
4 hours ago
Fair points. As mentioned, most of my transmuters have been octopodes and the dilemma isn't even there, for them.
Incidentally, +1 AC would be rather valuable for an octopode, early game, but I can see that being less of an asset for a merfolk.

He lies by omission, noting I've never won a game, thereby implying that I cannot. Which is obviously false, as my YouTube channel's extensive footage of deathless Extended gameplay demonstrates.

Eptagon is incapable of reasoning independently, and therefore must rely on social cues to form his opinions. That is why he argued against math and backed down only when outnumbered.

Wazzur's points were apt. To add thereto: An XL5 MfTm with -1 Spellcasting shouldn't use Ice Form as a substitute for bread and butter Beastly Appendage.

Eptagon's statement that an OpTm should do the same is even more wrong. UC improves tentacle slap chance per swing. This pairs well with a fast flat-damage branded shortblade. The sequence: pre-cast BA, enter combat, S2S, quick shortblade swings until initiate constrict, heavy longblade to finish.

Eptagon's claim that Blade Hands renders the helmet "unquestionably" better is a joke. MfTm should still use Polearms and Shortblades, for which Beastly App remains a valuable damage supplement. Blade Hands is situational due to its multiple drawbacks.

The real reason MfTm Amnesias BA is that he finds a helmet that adds something important besides AC.

Needless to say, Eptagon remains on my mute list.