Ups and downs for both:
3000 benefits:
Zorro slots for easy expansion.
Built-in scan doubler
Comes with 030 as standard
SCSI interface for CD ROM and hard drive
1200 benefits:
Many cheap accelerator cards available
PCMCIA slot.
IDE slot.
AGA.
In the end, for heavy expansion its easier to start with a 3000, since towering a 1200 is a fair amount of work, cost and fuss.
If you want RTG, its pretty much 3000 or towered 1200 anyways.
On the other side, its easier to fit simple IDE CF options to the 1200, you get PCMCIA for networking and file transfers, and you get AGA, which opens some software options.
