I think I'd go for it. Give me Zetterberg over Koivu in a points only. Sure he might explode having Parise on his wing, but there's an equally good chance that he doesn't mesh well with Big P and has another ~60 point campaign. Likewise, I'll take J Staal over Granlund. If there's a guy that's going to really pop this season I think it's Staal. It doesn't hurt that you've got his probable linemate in Semin also on your squad, and I'd probably keep Staal over him at the end of the year as well. So you're improving your keepers. You've got enough depth with your young guys that losing Granlund won't hurt so much, even though he's arguably your best prospect. For this year also give me Pitkanen over Gardiner. If Gardiner really surprises and does well then they will end up about even. But Pitaken usually nets around 40 points, so I think he's more reliable. He's also going to be on PP1 with Semin and J Staal, so you might be able to get a bunch of points off them playing together.
Based on projections it's a very fair trade, but I think that the Zetterberg side will come out ahead at the end of the year. If you're worried about getting older then I'd say don't do it, but it looks like you've got more than enough youth. Is there a farm system in your league? That was my first thought when I saw Granlund, Huberdeau, Tarasenko, Gardiner, Schultz and Baertschi on your roster. Or are you hoping that one of them will have a big year and justify a keeper spot while you let the rest of them go unprotected?
16 team, full keeper
19 roster players, 15 prospects
Stats: G, A, PPG, SHG, special teams assists, SOG, FOW, PIM, hits, blocked shots, +/-, W, SO, saves, s%, GAA, shootout saves
C: Crosby, H. Sedin, Boyle, Brassard
W: P. Kane, Voracek, Burrows, Ott, Filppula, Dorsett, Hudler, Bouchard, Zuccarello, Stalberg
D: Karlsson, Letang, Enstrom, Boyle, Giordano, Gorges, M. Stuart
G: Quick, Bryzgalov, Kiprusoff, Ellis
Prospects: Granlund, Atkinson, Pirri, Killorn, T. Johnson, Galiev, Corrado, Koekkoek, Moore, Rundblad, Bachman, C. Johnson, Desjardins