The GBA version pwns the Anthology version, IMHO. NO loading times, great new translation, updated graphics, greatly ported soundtrack and an all new dungeon with some new super bosses and super weapons. It's totally worth it.

However, if you have a DS and you want to buy V, you might want to wait a tad longer. Maybe they'll announce FF V for the DS, remade in 3D like IV. But if you absolutely can't wait, by all means, grab that GBA version. It's the best version of FF V there is (for now).

And about spells, you only need to buy them once and the entire party will be able to use em.