depends on how strong your team is... 5% water atk might make a difference if you're packing some really high assault... the "meta" players probably got a grid full of assault and pride at SLv20 to make that 5% matter...
Tiamat though... possibly blows those two out the water.... pun not intended...
Tiamat has more atk than most eidolons in the game, ups the rate of double (i'm seeing 5% more chance from comments), and provides a barrier... reiki is just 5% more water attack with around 2/3rds the raw atk as Tiamat and comes with a fire resist, while JF comes with the same things as reiki (more or less), but sometimes removes a charge from bosses and has slightly higher raw atk than reiki...
it's more a case of what you need... in my case, i rolled a ton of defensinve kami and not many offensive ones (hell, my SSR in water element is Aphrodite, Snow Raphael, and Sarasvati). Jack Frost with that pretty much lets me outlast a lot of fire fights and even some off element ones (although my wind and light teams is better for that). i still tend to use reiki as a friend (and sub considering my reiki isn't MLB) but in fire fights, i'm not dying anytime soon...
Assault grid won't matter for this, since modifiers just multiply each other. In other words, you should only be looking at the part which is affected - in this case, elemental attack. That's anything between 180% to 255% with double Jack Frost/Tiamat. Add 10% for double Reiki.I did some speculative maths earlier. No, Tiamat doesn't blow Reiki out of the water.
Yes. For overall damage. I believe I've personally said 80%, but that's just an arbitrary number - it's around there.
But. When comparing two sources of elemental damage, just compare it to other elemental damage. Because regardless of the other modifiers (such as Assault), the damage increase or decrease will be in the same proportion. As I said in my speculative maths, 10% elemental attack is 4-6% more damage for you.
How much 4-6% actually is for you will depend on stuff like your Assault grid, but the 4-6% won't change. So when compared to the ~9% boost of Tiamat, yes, it is higher, but not high enough that you'll need to feel bad for using Reiki.
even still, in terms of defense, tiamat should also be better than reiki... unless it's fire damage you take
reiki gives fire resist (more than jack but for 3 turns only as opposed to the entire fight), tiamat gives a flat HP barrier... this means tiamat is less situational than reiki int he case of being in sub slot of for using a water team in an off-element situation
Which shouldn't matter, since you can use all of them if you want. Especially Tiamat will be a sub-Eidolon for everyone who doesn't whale, due to having the highest stats of all event Eidolons. At least for the next year.
If you're fighting Fire, nothing can beat Jack Frost, really. But if you want damage over survival, Tiamat and Reiki are about equal. Tiamat is probably slightly ahead, but less reliable and doesn't work on skills.