Hm, well.. as I see it, you can make just as much electrum (and much more score) in T50 with a good rush as against FG's with a good anti-FG deck. However, winning upped cards is much cheaper than upgrading one by one. Pulvy/Eternity/Electrocutor if you plan on doing FG's, Arsenic/Vampire Stiletto/Druidic Staff, etc if rushing T50. I guess I see Arsenic being used in the largest amount of decks.
Fahrenheit is useless in the fastest fire rush (destroyers), Trident is stall and not as useful as the big three anti-FG weapons, Eagle's Eye is too quanta-heavy, Titan is too slow (or out of place in a Fractal Charger deck. don't need the vulnerability to perm control), and so on. They all have their uses in specific decks though, like flying Titans anti-FG deck, so it's all about how you plan ahead. That said, you can get all of them in T50, so I'd do what I did and farm that first. I used speed poison at first, so I got Arsenic.