Looking at traces real deep is pretty much the thing you do when you have othing else left to play with fwiw
For example, The Barr Report has been focussing on the little discussed traces for the past few newsletters. Brand preference is just a preference: its all pretty much the same.
The big selling point of something like Flourish or TMG is their chelator quality, which ensures that elements (say, Fe) stay in their available state for plant consumption. This also increases efficiency, so theoretically you shouldn't have to dose as often or as much. For example, TMG's directions state it only has to be dosed once a week, and TMG's recommendation doses much much less Fe than most hobbyists.
For what its worth, I switched from CSM+B+Extra Fe (Greg's old/discontinued "experimental mix") to TMG, then to dosing both of them together. If you dose every other day I don't think chelation quality matters so much, and personally my tanks run better with rich traces, and TMG is too expensive to dose richly alone. (You could say I supplement CSM+B+Extra Fe. I'm also cheap.) This is all nitpicking imo but I have some comparison pics if you want.
My suggestion is to pick whichever one (Flourish, TMG, CSM+B) is more appealing to you. There is no compatibility issues with your other ferts.
how does the flourish not go bad?
Dunno, but Seachem reps seem to be real prompt and informative with questions. Maybe they have an anti-molding agent or something in there. Maybe its something with the bottle. Maybe some bottles do get mold eventually.
Following Seachem's percentages of solution, it really is true that Flourish Trace is glorified tap water btw.
edit: typing while Purrbox posted.