I've now seen a couple of articles and posts comparing Avery Bradley and Stevenson, which seems like a natural comparison. I've seen comparisons come out each way.
What seems to be getting lost in the shuffle in the dialogue is that the players cannot be compared in a vacuum when talking about the Celtics signing one or the other. The Celtics did not have the same opportunity to sign each player. They had bird rights to AB, who was restricted, and no relationship to Lance Stevenson. That is big.
Let's pretend the Celtics desperately wanted Lance to start off free agency. They had some cap room, but would have to renounce all of their cap holds to use any of it. Goodbye draft pick and trade exception from the Kris Humphries trade. Goodbye possibility of the same with Bayless (didn't end up happening, but they wouldn't have known that at the time). Most importantly, goodbye bird rights to AB and knowing their cap situation, goodbye ability to match him as a RFA.
Even after renouncing all of those, they would have been just short of the money he ended up getting, which means likely cutting Chris Babb and Cris Johnson. They let Phil Pressey become guaranteed recently, so presumably they really like him and would have tried to hold on to him.
They wouldn't have been able to make the trade exception trade with Cleveland. Before everyone starts jumping up and down yelling that the exception is for teams above the salary cap, so they could use it, let me explain. What I really mean is they would have to sign him before using the trade exception. Once they make the trade with Cleveland, those players count on their salary cap and would have easily pushed the Celtics above the cap number keeping them from signing Lance. Obviously the trade exception expired right after the start of free agency.
In theory, the Celtics could have signed Lance right at the beginning of free agency and still had the ability to make a trade with the exception. My feeling is that Lance would have commanded a premium to lock him up that early. He may not have been willing to sign that early at all. Assuming he would command a premium, the Celtics would have likely had to also cut Keith Bogans, losing yet another asset. Even if they managed to get all of that done, there is nothing saying the Cavs and Nets would have waited for the Celtics to do it. It was clear LeBron wanted the Cavs to show him they could clear the space before he decided. Perhaps they move on to the Lakers or Sixers who both had cap space for sale.
The Celtics are unlikely to use their full non-taxpayer mid level exception this summer barring significant changes. Still, it is yet another asset still hanging around giving them flexibility. If they had followed the above blueprint to get Lance Stevenson, despite having a lower payroll overall and the ability to use the mid level while staying under the luxury tax, they would be unable to use the mid level exception.
So, in all likelihood, the choice for the Celtics was between Lance Stevenson and Avery Bradley, Cleveland's first round pick, Tyler Zeller, Marcus Thornton, 2nd round pick from Washington, trade exception from Washington, their mid level exception and some combination of Chris Babb, Cris Johnson and/or Keith Bogans.
Obviously the Celtics didn't know at the time they were making this choice what some of these assets would turn out to be and maybe they wouldn't have worried about a potential Cleveland trade since trade exceptions so rarely turn into useful assets anyway, but also possibly there were ten other potential trades that didn't happen that a plan of going after Lance would have precluded.
Just one other thought, much is made about the structure of the two contracts and guaranteed money in each. Lance's contract is shorter and with a team option because he's batty and the team needs to protect themselves. Avery's contract is longer because he is less of a risk to fall off of a cliff. The contracts' structures fit the players and are a wash in my mind. The yearly amount is the best form of comparison in my opinion.