Ooh those do look interesting. Haven't seen one before.
Just a quick thought though - this strikes me as being something you'll ever never put down or it'll be used a few occasions and forgotten about. It's quite a specific usage and technique. With that in mind, I'd say get the 35mm. It might get used more often - looking at the loading and usage instructions, I'd rather get it wrong a few times while I'm learning on a roll of 35mm than 120.
If you're keen to go into 120 in a fun way, think about a 120 Holga. Yes, they're plastic, have variable build quality. Lens quality varies, f-stops vary, and you may find some individual light leaks. But, I'm told, that's all part of the charm.
Personally, if I ever get into 120 format, it'll be with a Mamiya. Or that's the plan.
