The 1200 is the most practical choice. Somewhat decent form factor. AGA is nice for a few games, and tons of demos. It's easily networked.
BUT, the 2000's Zorro slots are a *major* bonus. A video card will more than make up for the lack of AGA if you'll be starring at text for long hours. Also, the 1200 desktop is a little messy to upgrade.
Personally I would choose the 2000. Zorro + unsurpassed (by the A1200) nostalgic value.
A2000, plus:
G-Force '030SpectrumIndivision ECS (all your games will be automagically routed through the Spectrum)
DENEB USB controller, handles your networking among other useful things
All this isn't a whole lot more (if not less) that what it will cost to upgrade the 1200. No AGA, but you'll have a much nicer desktop experience. And everything fits cleanly, no hacking up your case or having a zillion cables visibly hanging out from the back of your computer.
