So, general thoughts on other systems.
Shadowrun
So, if we were talkign anything other than 4th ed, I'd probably be fairly dismissive, but they really did clean things up in the latest version. It is vastly more streamlined, and the fact that I no longer have to buy spells at a specific force level is an oh-my-god improvement. It's drain model has been very influential on the development of non mana-based magic systems in many RPGs, and with good reason, and while we won't be taking from it directly, it would be unfair to say that it is not an influence.
That said, as a system, a lot fo the thigns that make it a strong match for Shadowrun end up being problems for Dresden. It's a spell based rather than effect based system, so it would require major retuning to handle the sort of effects that we see Harry do. Other key components, like Initiation and the Essence rating, just don't translate well out of setting.
Mage
Ok, when people say Mage, they tend to mean two different things: Old Mage, with subjective reality, and New Mage, with it's more firmly established cosmology.
The both use sphere-based magic, which is (or can be) effects based, which is potent and flexible. If I want to throw a bolt of fire at someone, the GM goes Ok, that will take this many spheres of this type and maybe this many spheres of this other type. While the specifics vary between the two games, the core principal is incredibly potent and flexible (as it is in Mage's grand-daddy system, Ars Magica, which was my first exposure to combinative magic).
The problem is that while the concept works, Mage operates at a layer of abstraction that is a bit too far removed from where things happen in Dresden. Pretty much all of evocation could be folded into Forces, and many of the other spheres are beyond the scope of where the focus for Dresden really should be. In short, to run Dresden with Mage requires neutering Mage, a solution that I think benefits neither party.
(That said, I just picked up Second Sight, the nWoD miscellaneous powers book, and it's nicely indicative that someone _could_ build something closer to Dresden than Mage within the nWoD system, but I've always been a fan of the new core rules, so that's no shock.)
Now, setting aside the specifics, can the _concepts_ of Mage be translated, perhaps with a more thematic set of spheres? Well, probably, but I wouldn't do it for a few reasons, most notably that it is almost _too_ effects based as a model. Old style Mage had Rotes, but they felt more like a system gimmick than any kind of way to ground play in magical tradition. New mage addresses that substantially, but it does so in ways which (rightfully) reinforce the specifics of the new setting.
However, I do not think that one can do a modern magic system without owing some debt to both Ars Magica and Mage, and we certainly won't be any different.
If people have other systems in mind, I can try to address them. I haven't played everything, but if I can reassure folks that we're not dismissing other approaches out of hand, I'll be happy to try. And just for reference, I love Mage, though I am definitely of the new rather than old camp for all the trouble that causes, and I just ran the first session of a Shadowrun game this past weekend and it went pretty darn well.
-Rob D.