Smart is a core player going forward, Hayward is not.
Both help you win games in their different ways.
I didn't vote in the poll - there's no reason why you can't keep both.
The OP said that we need a big or a shooter - Hayward is an excellent shooter; but more than that, he gets his teammates good shots.
@dannyboy35 mentioned that he's more concerned about Kemba Walker's knee. I share that concern. His game depends on skill and quickness with the ball; if he's out, Boston's offense shifts down a gear. This is a concern for next season, and the Celtics should retain some flexibility to add a point guard for the stretch run.
I like Wanamaker, but he does not create a lot of shots for his teammates. He's been turnover-prone, so it's maybe not even wise to ask him to try to do more to get his teammates going. He's got good size and discipline and gives up his body on defense, gets lots of deflections - solid team defender, and gets to the line (and of course he led the league in FT%).
What Kemba brings to the game is difficult to replace. Smart doesn't have Kemba's quickness - with and without the ball; or his shooting; or that all-important shot-creating for his teammates.
But no, don't trade Smart for Jrue Holiday, as good as Jrue is. They're different players; Smart fills a lot of holes, provides whatever is wanted and needed whenever he's on the court, and above all he provides championship-grade leadership.
He took a big step forward this year, brought his turnovers way down, shot a decent 3-pt% - but calling out Jaylen and the team in the Miami series was a great moment for him and the Boston Celtics. Nothing personal. I note with interest that they won the following game, and Jaylen was great.
Brad has said that Marcus does "the hard things" - that includes getting loose balls and crunch-time steals, but it also includes the mental part.
A big or a shooter (or a bench scorer, which others have mentioned)? I don't agree with that whole approach, I think it's the wrong way around. If you add anyone this off-season, let it be an improvement over an existing player, an upgrade - not a strategy to fill some theoretical slot. Get a good player, don't get someone to fill a role - let the coaching staff figure out how to make it work. It's hard to argue with how thoroughly they re-vamped the Celtic offense to fit their personnel this year; and the offense wound up better than the year before.
Boston was fifth (some say fourth, depends on how you calculate possessions) in offense and also fourth in defense last year. The bench was sixth in net points. Obviously the Conference Finals is nothing to sneeze at, either, and they even out-scored Miami despite losing four games to two.
As Danny Ainge has frequently pointed out, you can't just get "a rebounder" if he's going to make the team weaker in other areas. So trading Hayward or Smart - or both! - to get that 'serviceable big' is mostly going to make the team worse overall.
This off-season might be the trickiest of the Brad Stevens era. That last step is the hardest, and the step before that is almost as hard.
You're going for it all, so adding more rookies looks counterproductive. They'd have to be pretty good to justify more personnel churn and more starting over with new players. One of Stevens' unsung strengths has been his ability to find minutes for his bench players and his rookies; but the time comes when you just get about the business of winning; and it's a rare rookie who helps you win the third and fourth rounds of the playoffs.
Tatum, Brown, and Smart are your core guys. They're all defensive leaders and are dedicated, which you need your top guys to be.
Hayward and Walker are older and have both had injury issues. Hayward's all-around game is needed next year, especially if Walker has a lot of down-time. He's sacrificed his scoring to provide whatever the team needs; maybe this is why his game is undervalued. His contract runs out when Giannis' does, just sayin.