I am an environmentally-concerned artist that creates recycled cards
and stationary and would like to know if it would be better to package
them in cardboard boxes or tin boxes. What is the relative
environmental impact of each? or do you have a better environmental
solution?

It seems to me that since cardboard is biodegradable, it is more
environmentally friendly. Tin will eventually rust away to nothing,
but the cardboard would be long gone.

Tin does not rust. Tin artifacts have been found that are thousands of
years old. Cardboard is essentially wood pulp; if exposed to the
elements, it deteriorates and mingles with the soil. If I were in your
position, I'd choose cardboard for its environmental friendliness.
However, some commercially-produced cardboard boxes use plastic or
other synthetic material as a glaze or coating. You might want to
steer clear of that.

Huh? I was confused by the first link below, then the mystery is
solved by the second link.

String: rust tin remove

Sherwin Williams Paint FAQ's
http://www.sherwin-williams.com/FAQ/pro_exterior_product_sel_faq.jsp
..."Q. I have an old, rusty tin roof to paint. What do you recommend? 
A. First, determine that the roof is indeed tin, and not galvanized
steel. If it is tin, remove any loose rust with a wire brush; then
clean and rinse the surface to remove any dirt, mildew or surface
contaminants. After the roof is dry, apply a slow-drying,
rust-inhibitive oil-based primer followed by two coats of a quality
topcoat..."

But then, I entered"  [ "tin does not rust"  ]and found the problem. 

Notes on Metals and Common Alloys
http://sunny.crk.umn.edu/courses/AFSM/2243/Lectures/Notes%20on%20Metals%20and%20Common%20Alloys.doc
(BOTTOM LINE)
..."Tin-  This metal has been extracted for thousands of years. Pure
tin does not rust .  Tin cans contain less than 1% by weight of actual
tin..."

SO!  The moral of the story is if you select TIN, make sure it is 100%
tin, not an alloy.

Thanks for the heads up oh Pink one!

All very true, but I suspect that Greenquest-ga is thinking about
tin-plated iron boxes (like "tin cans", "tin" boxes for chocolates,
etc.).  Pure tin is too brittle to be made into thin-walled boxes.
Go with cardboard, unless you expect that the price of your work can
justify the expense of packaging in "tin" boxes, which will then be
kept for other use.
Environmentally, the production of the boxes should also be
considered, which also goes against the use of metal.