What kind of print for constant UV sun exposure

A client would like a photo that will be displayed in a window where it will be exposed to daylight practically everyday.

What kind of product should I be offering that can last fairly long when exposed to constant sunlight / UV rays?

Originally, I was leaning towards metal prints, but then I think there will be glare from the sun, and that would not be pleasant to people passing by.

This might be of interest
http://www.marrutt.com​/digital-ink-myths-2.php
The other point to bear in mind is photodegradation of the paper itself.
I dont have any experience of light fastness on printed paper but putting my ex textile colourists hat on I can see problems of light fastness of a print exposed to sunlight for a long period.Certainly if you had most dyed texiles exposed in those conditions you would get fading and generally some photodegradation,parti​clarly with natural fibres and paper is cellulose based,like cotton.
Even in automotive textiles where extremely high light fastness is a requirement you would get some fading and degradation.
Talk to the printer regarding light fastness and possibly consider rotating a number of prints to reduce the individual exposure time.

http://www.wilhelm-research.com/

René Damkot wrote in post #10266087
http://www.wilhelm-research.com/

Rene's link is a good start, but there's a lot of info on Wilhem. To get you started, Hahnemuhle makes a lot of papers (rag) with excellent longevity. Also, you did not say if it was framed or not. If framed, you can use UV glass which is not that much more expensive and will do a good job of protection.