My char is a cleric/ranger.

I'm stuck between shadow dragon scale armour and full plate mails.
What should benefit me in the end?

Do you actually plan to use your stealth ability for anything?

If not (you don't need to) you could easily slam on a l33t platemail. You could still keep the Shadow Dragon scale for encounters where stealth might be useful-.

Still, you could easily make do with Shadow Dragon Scale (and White Dragon Scale in ToB). It is really a quite awesome piece of armour. Nice black colour too.

Dragon Scale because you need its resistances and low AC.

Uhhh....Alson....do you have a Mac or something?

Little Known Fact: In my game, Full Plate+1 allows all Thief skills (no penalty, either!) except Stealth. Does the same thing in BG1, too.

strange...chain mail seems to prevent all thief skills except for detect traps...

Not strange at all: Wearing any armor that is not usable by a pure Thief will negate most Thief skills. (Detect Traps & Detect Illusions are non-armor-dependent, since all you're doing is looking around.) A Fighter/Thief in regular Chain will have most of his skills blocked due to the weight & clumsiness of the armor, but a Thief in Elven can still use them, although with some penalties.