Peak Smalling, say around 2016 was a far better defender than Maguire has ever been. Now, it has proven to be the correct call to move on from Smalling as his level has dropped massively due to his injuries in the last 3 years but Maguire has never been part of a strong defensive side. The hype around him was driven by those eye catching runs forward with the ball, which would become, predictability, pointless at a top club and his aerial prowess, which is best suited to playing a team who defend deep and counter attack.
Set up higher on the pitch and he is a major problem.
I've a different take on him as a defender in the sense that he is a proactively one who wants to be touch tight on his man and physically restrictive - when he first got here, before his confidence was eroded, he was all about breaking rank and meeting his man high - obviously with his slowness and turning circle, it's imperative he takes either the man or the ball, but never letting both go past, but he needs a sweeper keeper behind him and a defensive partner with recovery pace to play in that manner.
His poor judgement and reading of play conflicts with the game he has, however, which is why he gets done so often - it's not because he's slow and lacking mobility [alone] that he gets done, it's that he just doesn't read play well enough to play in the way he wants at a club with our aspirations. It's eye-catching at smaller clubs, especially when pulled off with ball-winning and immediately carrying the ball up the pitch and into a useful, progressive pass, but Maguire is not Koeman or Pique let alone Sammer or Beckenbauer, so he fecks up more often than completes the action. Combine that with a loss of confidence and being unfit, and it's an obvious recipe for disaster.
By contrast, England play as negatively as possible and enshroud him with bodies and play a deeper line on top - he looks a different player there but also not one playing the game that earned him his move or had City, and their ball-playing requirements, interested. I really don't think this is a perquisite for him, however, but playing that why obviously protects him and basically prevents him from ever being exposed except in one-on-ones with superior attackers where we see his level as a pure defender.
Smalling's injuries are fair enough, but outside of his ridiculous penchant for grabbing shirts in the box and his often spoken about technical issues, he's easily a better defender than Maguire. Reads play quicker, more aggressive, physically superior, faster, stronger, taller, mentally superior. Even in decline I'd say just there, Maguire's very best just about draws them equal. But it's by the by, I guess, especially with Smalling now being near over the hill in his own right.
Just madness that the outlay has been such a poor spend - at half the price, taking Maguire for what he is, warts and all isn't difficult, but for a line-leading captain, in the elite price bracket, he's been a bad spend with his average and poor games outstripping his good or great ones by some distance.