Well depends what you mainly want to use it for?
The A4000 is the best of the two machines.
Both Amiga's have AGA graphics capabilities but the 4000 has a faster processor and memory.
Also the 4000 has a standard Zorro busboard for graphics cards etc but can be expensive.
The 1200 doesn't have a Zorro busboard so you would need to acquire something like a mediator from elbox.
www.elbox.com