Go for the Sam! If you can afford it, it's a great idea. For starters, you wouldn't be able to run OS4.1 on the PPC 1200, because the last version that came out for it was 4.0. The Sam will run OS4.1, 4.2, 4.3... as they come out. Currently OS4.1 is the latest OS for it, and it looks incredible, and has features the A1200 version just can't do, like compositing and the 3D environment. Also, the Sam is new, and easy to get. You could spend months trying to battle your way through eBay to get an A1200 PPC accelerator, and by the time you get it up to similar specs as the Sam (Radeon Graphics card, PCI, USB, Ethernet, 512MB RAM, 16bit Sound...) it could cost you well over what a Sam would cost, and you'd have to fit it in a big ugly tower. The Sam is so small that you can rip out an A600 or CD32 motherboard and replace it with a Sam and have room to spare in there. Or simply get one of the many awesome and cute MiniITX cases that are available from PC websites or shops.
One more point to remember, if you buy a Sam, you're actively supporting the Amiga market and the future of the platform. The Samantha is the new Amiga, and was designed from scratch to be a new platform for AmigaOS4, as well as having applications in the embedded systems market and running Linux too. And she's cute too, only 17cm X 17cm, and hardly uses any power compared to a PC. She doesn't even need a fan, so if you use a solid state hard drive or a Compact Flash drive in there, she'll be completely silent too.
Sam wants to be your new girlfriend, why don't you go ask her out.