The team would have been better ON PAPER this year with Spezza, but that doesn't account for how trade talk and the rumour mill surrounding Spezza might effect things as a distraction. It was the right choice to move him, because a year of Spezza was worth giving up at this point in the Senators progression to get back Chiasson, a 2nd, and Nick Paul.
In an alternate scenario where Spezza didn't want to move on, I can't see how they could fit him in long term without him being willing to take a big discount. I don't see the team being better long term with Spezza re-signed if it means shipping off other players as they become more expensive in order to accommodate a 7.5M cap hit for Spezza.
Ultimately, the team would be better this year. on paper. Keep in mind, Legwand would likely have not been signed if Spezza stayed. Spezza would have helped a lot with the power play, and Ottawa could have probably rolled with three very strong lines.
It's impossible to say where the lines would be, because everything came together with time during the season, but in no particular order, this would at least be a possibility if they kept Spezza for this year....
Hoffman-Zib-Ryan
Mac-Turris-Stone
MM9-Spezza-Lazar
Condra-Pageau/Smith-XXX