The advantage of an A2000 is that you get Zorro 2 expansion slots. You can get these as well on your 1200 but then you need a tower and a busboard. Zorro 2 is slow though... The 4000 is a Zorro 3 machine and that is faster.
But you can also get some 1200 busboard that give you (semi) Zorro 3 slots, but then you also need an A3000/4000 type accelerator. But maybe the best option for 1200 is the BVision graphics card but then you need a Blizzard PPC card AFAIK...
Confusing heh? You could read more at
www.amiga-hardware.com There is a lot of info there about A1200 busboards and accelerators.