Wouldn't Scientific be good for 100K as well? Or is Agricultural more important? Might be an interesting experiment to compare a rel+agri with a rel+sci 100K game.
In Conquests, Scientific isn't nearly as good as Agricultural for 100K. It isn't quite as good as Religious. Having recently tried 100K with Sumeria, I would say that the Celts are significantly better than Sumeria, but that Sumeria is significantly better than Babylon. The reason is that you want the ToA, so you get free temples, and it expires with education. You will never build universities (assuming you go the usual way - ToA and Feudalism), and you will pop-rush like crazy.
(In PTW or vanilla, the 100K problem is significantly different - no agricultural civs, no ToA, no Feudalism, no SGL - and none of this applies.)
If religious: You get cheap temples (of little use, as the ToA gives free ones) and cheap cathedrals (nice, especially after you know gunpowder so you can use muskets as an intermediate step in rushing). Short anarchies mean you could revolt twice (republic, then feudalism) if you wanted (I don't think it is worth it), but even if not, since you don't gain culture in anarchy, this is helpful.
If not religious: expensive cathedrals are a pain, as there is no 140 shield build available: you have to have 4 citizens remaining with 120 shields in the bin to rush it. They cost an extra 4 citizens, but the lack of an intermediate build means that you must have an additional extra citizen that doesn't get sacrificed.
If scientific: You get cheap libraries (nice) and cheap universities (useless). You have better chances for SGLs - since you want both pyramids and ToA, this is nice. It is of less value if you are going to start a new game if you don't get a SGL immediately.
If not scientific: expensive libraries cost 2 citizens more than cheap ones - not a big deal. Without ToA, universities would be a problem.
If agricultural: You get extra food. This lets you expand faster (nice, especially on lower levels where the AI won't help you). More importantly, your population grows more quickly, so you can pop-rush sooner. Growing in 7 turns instead of 10 gets every improvement a lot earlier - it takes 30% fewer turns to get your buildings done. This also gives more buildings that double their cpt. Of course, if a town has more food available, the extra food from being agricultural doesn't help as much, but you will have many towns where the food supply is somewhat limited.