There seems to be a fair amount of controversy on this topic, however, I am planning on printing a 30x30" 3D design on aluminum, and I can't decide if 300 or 600dpi would work best. Or even higher. The only problem is that after rendering for most likely a week at 600dpi, I'm not sure if photoshop would be able to even handle making small edits to the design. Also, printing on aluminum is pricey enough where it makes me anxious to print at a quality that might just pass as "okay". This is for an art show/gallery, by the way, so people will be viewing it very closely I am sure.
I hate to say this but you should really ask the printer for this; he'll tell you what's the max he can print at.
If you speak with their prepress department and the printer operators, they'll really tell you in details what's the best process.
Don't waste your time with the sales department, (no offense to salesmen) they usually repeat the same cassette and the care they put in your project is proportional to the commission they get from it. Small artsy projects often annoy them in fact, so speak with the prepress department, they might in fact be thrilled by this challenge and happy to help you!
3D is long to render, but if you're not sure it's always better to have a higher resolution when possible since it's easier to decrease if you change your mind than increase it, as you certainly know.