I agree I'd rather have Butler or Leonard. Middleton and Harris are close, Middleton would be cheaper, but unless we move a guard, not sure if he would be a good three, were Harris could play some four.
Middleton and Harris really aren't close.
Middleton is already one of the best shooters and defenders at his position in the NBA.
Harris is (so far) an entirely average shooter and well below average defender.
Middleton is a far, far superior option right now...and likely cheaper as well.
Middleton is a superior shooter and in some ways a better defender, but he has no interior scoring skills. He does not drive to the hoop, he does not post up and he does not get to the FT line. He took less than 18% of his shots within 3 feet of the rim and less than 29% within 10 feet. He is a textbook 3-and-D player. A very, very good 3-and-D player, but that's all he is. And on D, he is not so great at defending on the block.
Harris is a much more versatile scorer, with both perimeter shooting and various skills at creating points from different points of the floor. He's an excellent post-up player and can get to the FT line. He took 29% of his shots within 3 feet and finished them at a ridiculous 68.7% efficiency. Even though he's not quite the elite 3PT% shooter, his more versatile scoring repertoire has him as almost as efficient overall on scoring attempts, posting a 55.1% TS% compared to Middleton's 56.3%. That difference is basically in the noise.
Consider for a moment that both have raised their TS% each of the last 3 years, demonstrating steady improvement in efficiency as they've gained experience. But Harris is a year younger.
Harris is not an elite defender, but he is not bad at all. He can adequately front most SFs and all but the faster SGs on the perimeter. Plus, unlike Middleton, he can at least hold the block reasonably against bigger SFs like the Carmelo's, the Paul George's and Lebron's of the Eastern conference. And he's a much better rebounder, which is a critical part of defense. And, again, he is very young. Defense is one of the skills that can be coached and improved with experience and a good system.
I think your assertion that Middleton is a "far, far superior option" is very questionable. He's also looking more and more likely to remain in Milwaukee.
To say Harris is "not bad at all" defensively may seem true subjectively, but the objective raw hard facts tell a very different story.
The variation between Middleton's Defensive RPM (+4.10, 8th in the NBA among all positions) and Harris' Defensive RPM (-2.2, 408th in the NBA) really is so drastic that it's completely impossible to ignore. Middleton ranked in the top 1% of NBA players, while Harris ranked in the bottom 15%.
Whether you go off 'eye test' or stats is up to the individual, but since it's completely impossible to compare subjective opinions, I tend to prefer the use of stats for comparison purposes when it comes to things like defense (that are hard to quantify with simple stats).
If somebody else can quote me any other defensive stats that tell a different story about Harris (opponent per, opponent shooting percentage, etc) then I'm perfectly happy to look at it with an open mind and consider those arguments, but until somebody can present some type of argument other than "you can just tell by watching him" I won't be convinced.
Also Middleton is more than just 3+D. Only 31% of Middleton's shots were three point attempts - that means 69% of his field goal attempts were
not threes. The majority of his offense actually came from midrange (40% of attempts from 10 feet to the three point line) and he shot some of the best percentages in the league on those shots.
I will concede that he's not much of a shot creator and is really more of a spot up guy (Harris is better in that regard) but then Harris' superior ability to create offense hasn't really helped him much since their outright scoring numbers (16 PTS Per 36 for Middleton, 17 PTS Per 36 for Harris) are ultimately the same.
Another concern is that Harris (as you said) gets a good chunk of his offense inside three feet, where he shoots a very high percentage - yet his overall FG% was still no higher than Middleton (who is primarily a spot up shooter).
Also even though Harris does (as you say) get to the line at a far higher rate than Middleton does (34% vs 19% last year) this difference hasn't led to as much of an advantage as you'd think. Harris averaged 2.9 FTM Per 36 Minutes, while Middleton averaged 2 FTM Per 36 Minutes. This probably has a lot to do with Middleton shooting a significantly higher percentage 86% vs 78%) from three.
Likewise the free throw disparity hasn't given Harris an advantage in terms of scoring efficiency, since they both averaged 1.22 Points Per FGA.
Yet if the game is on the line, and I need somebody to take the last shot, I'm going have more trust in Middleton (who shot a higher percentage from almost everywhere on the court, and from the foul line) then I would on Harris.
Once you factoring in Middleton's better passing skills, I'd say that two two guys are pretty close in terms of their overall offensive contribution, but I just feel like the one of out biggest needs right now (along with a rim protector) is a lights-out shooter that absolutely demands respect from the defense. Ray Allen offered us that dimension before, and it completely changed the way teams defended us. It's a similar deal with Atlanta, where the addition of Kyle Korver (and what he does for attracting defensive attention) has completely changed the way defenses play the Hawks.
I feel like if we can't get a legit star (which neither Middleton nor Harris projects to ever be) then the only other way for us to become really dangerous is if we fill out needs - those being a lights-out shooter at the SG/SF spot, and a high level rim protector.
While Harris is a capable three point shooter these days (much like Bradley and Olynyk are for us) he's not enough of a threat from three to the point where he forces teams to specifically game-plan around him.
Great shooters (Nash, Curry, Ray, Reggie) scare the Beejesus out of opposing teams - every second of every minute on the court you need to know where they are.
On the other hand 'casual' scorers (as I like to call them) like Jeff Green, Avery Bradley and Tobias Harris are usually more of a defensive afterthought. You know they are there, but you don't really worry THAT much about them.
Again each to their own, but IMHO Middleton would improve this team a LOT more than Harris would.
That said none of us know if we have a shot at Middleton at all, so that's neither here nor there. But if we did, my money would be 100% on Middleton.