I have to decide between Jar of Eyes (ASE Faction Event) and Flammifer (Raid Boss Weapon) since I do not have enough Gems for both.
Jar of Eyes seem to be some kind of Mountain Crusher 2.0 (and being blue - so Titan-Synergy) while Flammifer being more situational but with insane Sheggra-Synergy (for example).
Which one do you prefer?
Both good, but jar of eyes is the far superior weapon IMHO
Absolutely get flammifer.
Jar of eyes is coming back in the next ASE event. Flammifer, being an event weapon, is a one shot thing. And it’d a damn good weapon later on, so bag in and grab it!
Flammifer is great with yao guai/tai pan/queen titania, for example. And with red troops in general, and upcoming mythics (like Phoenicia).
^^^ totally forgot that jar of eyes comes back around with delve events - duhhh
Yeah, get flammifier as @BelethS suggests…
You are talking about this discussion I guess:
So my decision is clear - thank you.
I just can’t see why the one is the one shot and the other isn’t. Is it safe to say: Faction Event Weapons will come back sometime?
Not just sometime, Delve Tuesday cycles through all released delves, no randomness involved.
Yeah faction weapons will come round as the delve events appear - iirc we haven’t had a ‘proper’ delve event on switch yet?
That’s clearly arranged on Switch - we do only have two of them. 
I don’t know exactly. I didn’t care much of the Underworld in the past…
Concur with others: get flammifier from the raid shop as a priority, since you don’t know if you’ll ever see it again if not (currently, you won’t).
Jar of Eyes should also be in soulforge today, if you have the spare jewels and diamonds, you might want to consider craft it. This is one of my more commonly used hero mana generation weapons (I have it at +8 across the board so it can synergize with as many teams as possible and generally use a different troop to sustain or create storms) across all my accounts.
Delve weapons and raid/invasion weapons have similar craft costs, but the raid/invasion ones only take 250 gems to unlock while the faction one takes 410 gems. Since you only have two factions on Switch, you could wait the 2 weeks and drop the 410 gems next time, but I still consider soulforge for delve weapons a “less expensive” investment than the event shop, especially if you don’t plan on running the delve event, whereas soulforge for raid/invasion weapons I consider “more expensive”, since the shop offers a better cost/benefit ratio (a new troop not available elsewhere at t2, which is 100 gems anyways, and the weapon for 150 more or 250 gems in total).
You can get Jar of Eyes again. You will never get another shot at Flammifer.