If it's a base A2000 you own, both the 4000 and 1200 are a big step forward.
It's not the 'extras' you should worry about too much: CD-drives, harddisks and the other industry-standard parts can be picked up rather cheap, both used and new.
What matters is what you're planning to do. I'd suggest you take a piece of paper, write down whatever you like to use on the machine in the near future and then look at the availability and pricing of add-ons. Think about PCI-bus-boards, ZORRO expansions... the lot.
Keep in mind that the A4000 has been targeted to the serious user, but for the 1200 far more expansions seem to be available.
If the 1200 comes with a zorro-expansion board I'd go for that.