Let me try putting it a different way. Wayne in LA played on a weaker team than Wayne in Edmonton, and that team didn't accomplish as much. Does that mean that Wayne in LA was a better player than Wayne in Edmonton? Not necessarily.
Mats played on a weaker team than Pavel, and that team didn't accomplish as much. Does that mean that Mats was a better player than Pavel? Not necessarily.
I'm not saying Sundin was better or worse than Datsyuk (I think it's very close), just that we need to look at the player. If Mats was better, it wasn't because he was on a weaker team, it was because he was better.
I think too much stress on their respective teammates just weakens the argument.
Very few could compare to Wayne, not Datsyuk, nor Sundin...we need to look at Lemieux, Orr and Howe in order to compare to Wayne. It's definitely easier to focus in on stopping Wayne while he played for a weaker Kings team. Messier was the 2nd line centre with the Oilers playing behind Wayne...imagine that.
It's arguable whether Datsyuk is a better player than Sundin. Here are 10 years worth of their stats in their most productive years for their respective teams. Of note, Sundin played many years in the clutching and grabbing era, where he was consistently mugged and draped over by opposition players. Those Leafs teams also weren't very deep, so the opposition could simply focus on stopping him. Sundin consistently delivered more for a worse team over those years. I would argue Sundin is the better player.
Sundin:
Year: total points, team GF (goals for), percentage contribution to team GF
1995-96: 83 pts, 247 goals, 33.60%
1996-97: 94 pts, 230, 40.87%
1997-98: 74 pts, 194, 38.14%
1998-99: 83 pts, 268, 30.97%
1999-00: 73 pts, 246, 29.67%
2000-01: 74 pts, 232, 31.90%
2001-02: 80 pts, 249, 32.13%
2002-03: 72 pts, 236, 30.51%
2003-04: 75 pts, 242, 30.99%
2005-06: 78 pts, 254, 30.71%
Datsyuk:
Year: total points, team GF (goals for), percentage contribution to team GF
2003-04: 68 pts, 255, 26.67%
2005-06: 87 pts, 301, 28.90%
2006-07: 87 pts, 252, 34.52%
2007-08: 97 pts, 252, 38.49%
2008-09: 97 pts, 289, 33.56%
2009-10: 70 pts, 223, 31.39%
2010-11: 59 pts, 257, 22.96%
2011-12: 67 pts, 239, 28.03%
2012-13: 49 pts, 122, 40.16%
2014-15: 65 pts, 231, 28.14%