I agree on all the sell-high, buy-low stuff but I'd still give Duchene gladly to aquire Smith.
Smith's numbers may drop, but they'll remain very solid IMO. So you have yourself a relatively young, unquestioned starter for a defensive team. What's not to like?
Duchene on the other hand faces competition on both C and LW (Stasny, Landeskog, etc.) so he could end up on the second line. And keep in mind that your league counts many peripheral stats - Duchene isn't exactly a beast there....
So you'd give a rather one-dimensional potential second liner to aquire an entrenched starting G. Great thing.

__________________
12 team H2H dynasty
G, A, +/-, PIM, PPP, SHP, SOG, FW, HIT
W, GAA, SV*, SV%, SHO
C,C,C,LW,LW,RW,RW,W,D,D,D,D,Util,Util,4xBN,2xInf

Team:
C: Stamkos, Tavares, Kopitar, Hanzal, Legwand
C/RW: Backes, Pavelski
LW: Lucic, Landeskog
LW/RW: Lupul
RW: Clarkson, Hornqvist
RW/D: Byfuglien, Burns
D: Letang, Streit
G: Smith, Bryzgalov, Hiller, Fasth
IR: Kovalchuk

Farm:
S.Bärtschi, M.Foligno, E.Kuznetsov, J.Morrow, N.Niederreiter, T.Tatar