The difference between Hossa and Heatley in this time span isn't enough of a difference that makes the Senators win the Cup. The Senators best chance for a cup would have been in 05-06, when a healthy Hasek makes a huge difference over a rookie Ray Emery.
Hossa had a cap hit of $6,000,000 in 05-06, compared to $4,500,000 for Heatley. This was the season when the salary cap was at $39,000,000, and the Senators were a cap team this season. If Hossa stays, then someone else has to go. Are the 05-06 Senators better with Hossa, or a combo of Heatley and Havlat/Smolinski (two big salaried players who were let go in 2006 and one would likely be gone if Hossa stays in Ottawa in 2005).
The 06-07 team ran into a much better Ducks team. Again, Hossa makes $1.5 million more than Heatley in this season, which has its ripple effect. Heatley was pretty dominant in the playoffs until facing Anaheim. If Chara stays (a big if, apparently a reason he wanted to leave was to be the captain of a team, which wasn't happening in Ottawa as long as Alfie sticks around), then Redden is likely gone He had a good playoff run in 2007. If Chara and Redden resign with Ottawa, then maybe he gets his wish as Alfredsson and his cap hit are sent out the door (remember there were rumours of Alfie being dealt early in the 06-07 season after the team had a slow start). Alfie was the best Senators player in the post-season, and one of the few who showed up in the finals.
The 07-08 team was a no-show in the post-season. Does Hossa change this? He wouldn't have been much worse in the post-season than Heatley. But the entire team was bad after a great start to the season. I don't see them moving past the first round with Hossa on the club over Heatley.