Are you looking to win your pool now or later? I haven't crunched the salary #s, what is the price point of each package?
I think Backstrom is more valuable than Datsyuk in a keeper league based strictly on the fact he's a comparable PPG player and roughly 10 years younger. Datsyuk may outpoint him this year though.
Purcell at 27 is in the prime of his career and certainly will gain you more points than the younger Granlund. I love Granlund's upside, but Purcell is in a situation where it's not unthinkable he posts 65+ points regularly and it may be several seasons before Granlund even gets to that point.
I prefer Kulikov to Ellis [I hate the way NAS is run and coached], but Ellis is still a talented kid.
BOTTOM LINE...I think this improves your F this season and likely long-term as Backstrom is really the center-piece of this deal. I think this may push you to trade from your deep F core to compensate at D though [time will tell].
I'd probably do this.
10 Team [keep 10 + 4 prospects]
Scoring F=1[g/a]; D=2[g], 1[a]; G=2[w], -1[L], 1[OT], 2[SO] Top 12 F, 6 D, 2 G count
Forwards
Crosby, Ovechkin, Backstrom, Wheeler, Byfuglien, Hornqvist, Simmonds, Ladd, Zuccarello, Ribeiro, Desharnais, Semin, MacArthur, Moulson, Purcell, Soderberg, Dupuis
Defense
Keith, Goligoski, Carlson, Krug, Trouba, Streit, Pouliot, Ceci, Nurse, Martinez
Goalie
Fleury/Price/Rask