Rubicund cells are widely agreed upon to increase your quality drop rates. (Because help text).
That said, doing about 2k back to back (1k with 1k without), this is just a slight increase. I mean, its FFXI. We pay hundreds of millions for 1% increase.
Cobalt cells give inventory -1~X . They don't increase your drop rate of things like cinder/dross/hmp/skin/sap/etc. They increase the number of slots of "junk" you get. I used these religiously for thousands of pops. No noticeable difference in the number of worthwhile drops. Overall may get a 100k or so more to vendor if you have the bag space.
((Cobalt can be useful on a mob like Bismark, where Angel Skin is one of those items. On the HMP pouch carriers, well.. enjoy all the potions.))
eh, I have all the Key Items, I believe, and even when using Rubicund Cells (because my inventory cannot stand 36 rounds using blues as well) and STILL have gotten a chest of 4 Petrifacts.