Raid eidolons are some of the worst, don't use them unless you have no other option. Story exchange eidolons are just as bad but they can at least provide you with battle buffs.

It'll take less effort to max out a R eidolon that'll have decently good stats for beginners to use than it would take to level a raid eidolon to be worth using and only be slightly stronger than that R despite being 30 levels higher.

For example:
Werewolf, a dark R eidolon, easily farmable from certain story missions, 900 ATK at Lv60 (max) and gives 30% Dark ATK boost when maxed if using as main.
Dark Disaster, the dark SR raid boss eidolon (or any disaster works, they all have the same stats), 1000 ATK at Lv90, that's 30 levels worth of enhancing just for 100 more ATK and even at max break, it's only 20% ATK and HP. And this is assuming you can even manage to fully break limit it before you get something else worth using.
First off let me say that i have little experience with the Rare eidos so i can't say for certain about them. But let me just say that 100 extra attack is a lot more than that. I'm not exactly certain of the calculations but if I'm not misunderstanding the formula that 100 attack will have all the % increases from all the Assault weapon and the Eidos. Every little bit helps. Also don't underestimate the extra HP. While the dream is to just have a boat load of attack to annihilate the enemy most beginners don't have that so the extra HP is extra survival/chance to kill the boss. Also you will get the disasters while you are farming the weapons, which are necessary for the extra stats that most beginners don't have. Am I saying that the Disasters are good? No, by no means are they optimal. But if you don't have good options they may be the best of the pile that you have. Also most experienced players will have event eidos that are vastly superior to the Disasters. So this is aimed at new players that want to run all elements. My advice remains as i said earlier. Try to get 2 elements, your main and one to cover your mains weakness (if there is one). Hopefully luck is on your side and you get good Kamis and eidos. But if not the disasters make a decent bandaid.
This is just for new players though and completely based on my experiences.

My AP per chest rates are:
Beginner - 2.5
Standard - 2.4
Expert - 2
Ultimate - 1.70
These are probably not too far off. However the big point is that chests are not equal. 2 bronze chests might be better than 2 silver chests depending on what you need and what they drop.

This all being said the best thing to do is experiment. Find what works for you. This is what I've found with my experiments. Sometimes more attack is not the answer. Just look at the ragna Ixion. I'm not sure about anybody else but i doubt there is anyone currently that can just use lots of attack to kill it. It takes strategy and the ability to survive. Doesn't matter how much attack you have if you can be nuked out of existence.