I've never really seen Backstrom play, but I thought I remember someone posting that Nilsson's numbers in Sweden were better than Backstrom's at the same age. But, I'm sure Backstrom was drafted 4 for a reason.
Statwise it depends what age you're looking at, see below for the stats.
League Year GP Pts
at age 16
Backstrom:
J18 Allsvenskan 03/04 6 14
J20 Superelit (SWE) 03/04 21 8
Nilsson:
J18 Allsvenskan 00/01 4 9
J20 Superelit (SWE) 00/01 18 36
WJC18 01 6 2
at age 17
Backstrom:
J20 Superelit (SWE) 04/05 29 34
Elitserien 04/05 19 0
WJC18 05 7 5
Nilsson:
J20 Superelit (SWE) 01/02 21 31
Allsvenskan South 01/02 14 5
WJC18 02 8 5
at age 18
Backstrom:
J20 Superelit (SWE) 05/06 1 0
Elitserien 05/06 46 26
Nilsson:
Elitserien 02/03 41 21
WJC 03 6 6
At 16 Nilsson had the better stats, 17 is very comparable, though I'd give Nilsson a slight edge due to a better pace in the J20 league and he put up some points in the adult league, though it's hard to compare Elitserian to Allsvenskan. Then at 18, the points are again fairly comparable, but I'd give Backstrom the edge there.
Though, this is not to say that I think Nilsson will be better. If Backstrom continues to progress as he has, he will likely be the better player, and either way while they have similar offensive numbers, Backstrom's all around game is superior, so I suspect he will be the better player. But I didn't want you to look like you had no basis for comparing the two