For RM2/RMC, I would recommend the Alchemy Companion. In the first place, it was written for RM2. In the second place, it has a lot more creativity and variety in the professions and lists offered (TC has three professions, but the Channeling and Mentalism Alchemists are but minor variants on the Essence Alchemist. The AC professions are much more distinctive, to the point where you really wouldn't use all of them in the same setting.) Thirdly, they both have some parts that just don't work and needed more editing, so its a toss up there.
You get more bang for your buck with Alchemy Companion. Not only is it a thicker book, but it doesn't repeat itself. TC rather disappointed me with the degree to which it was padded out. You get magic item treasure generations twice: once with default values and again with a couple of blank spots to fill in if your assumptions vary from theirs. You also get a good chunk of the book filled with magic item descriptions, many of which are just "updates" of items you already have if you have C&T and C&T II, since the RMSS creature book is just creatures, no treasures. I'm not saying that you wouldn't get anything out of TC, but you'd get a lot more from AC. If you want a RMSS/FRP book that will give you a whole bunch of new material, of high quality, get the Construct Companions (golems, constructs, and much more).
Treasure Companion: 144 pages. Minus title page, TOC, index page, three pages of ICE ads, three pages of internal section TOCs, 10 pages of redundant treasure tables, eight pages of check lists that are just the names of spells on lists organized by type (five pages of which aren't even for TC, but for the Arcane Companion!), and four pages of updated skills sheets for RMSS, this leaves 113 pages of potential interest to you, without yet even considering the repetition of material you almost certainly have from RM2.
Alchemy Companion: 208 pages. Two pages ICE ads, title page, two pages TOC, and an 11 page index to spells and spell lists (all for this book and likely to be found actually useful), leave 192 pages of alchemical goodness, with some possible minor duplications of material in other RM2 non-core books, including the Spirit Runes material that prompted the thread. I don't have the Magus in front of me, but if the two versions of the list are identical, there is certainly room to add some more spells for additional effects. Ultimately, what you can and cannot do is up to your GM. He might allow you to develop Spell Mastery to use the OB rune spell as a general item bonus spell. There's so many available options with RM2 that's it is very hard to advise on someone else's campaign. Talk to your GM.
