I am more impressed with Ars Magica conceptually the more I poke around, and I am also more frustrated. Third Ed certainly doesn't deserve any awards for well organized, or clarity of writing. Apologies ahead of time for using old D&D spell references, I am not up to speed on the local grimoire yet.
Because I am joining a game thats been running for nearly 10 years real time I've been granted an item with 20 spell effect points invested into it, or 2 15 point effects (as well as extra arts/spells), so now I am considering a focus vs a talisman vs a bunch of invested device vs a bunch of lesser items). The more I look at the item creation rules the more I think that is either pretty generous, or not enough information to make something... Though as I read the item rules very carefully I see that spell foci are material components (sometimes optional, and sometimes reusable) that are spell specific.
It looks like attunements come from the form and material table... so an oak staff would get both the bonuses for being a staff and being made of oak if both attunements had been made?
So a talisman get you form and effect bonus via attunement, but is a, uh... material link to the creator and user?... requires a significant amount of enchanting effort (time and vis).
A spell foci gets you bonus(es) to cast a specific spell without any actual enchanting happening
An invested time could be a spell focus, and is a talisman that is capable of casting a spell on its own (ie wand of healing+ the potential of more spell effects).
...and finally lesser items could be spell foci, cannot be a talisman, do not provide a material link, and do produce a single magical effect (disposable(?) wand of magic missile)
How did I do?