Magnezone is the better choice. Probopass might have excellent defenses, but it has horrible typing. The Rock typing is what makes this horrible, as instead of resisting Grass and Steel type attacks (attacks which you'll see used most by Ferrothorn, which means something Magnezone can comfortably switch in on and set up Substitutes and Charge Beams is something Probopass has to hit right away. It also makes switching in on Scizor a more difficult task) it takes neutral hits from them. It also adds on a Water weakness and a x4 Fighting weakness, making switching in on Scizor even more dangerous than doing so with Magnezone, as Superpower can easily KO Probopass, while Magnezone can survive a wrongly predicted Superpower as long as it isn't a Banded or Life Orb Scizor (and has a very good chance of surviving even the Life Orb Scizor with HP Investment). While I know these are the most common Scizor, there is still the Scarf Scizor, which sees some use. The typing of Probopass is awful, in comparison to the excellent typing of Magnezone, which sports the most resistances in the game (along with the rest of its family).
Magnezone also has the advantage of being able to do something for the team besides trap steel types thanks to those solid defenses (combined with the aforementioned excellent typing), and an incredible Special Attack. This allows it to hit hard with Thunderbolts (or powered up Charge Beams if that is how you roll). Probopass does not hit hard, even if you decide to give it Special Attack/Attack investments. So once the Steel type is down (granted that Probopass does not fall in combat to said Steel type), Probopass becomes fairly useless for the team. As Slade said, it can set up Rocks. But so many other Pokemon can do that anyways, and do it far better (Ferrothorn, Heatran, and Mamoswine are three that come to mind). The fact that Probopass is useless outside of Steel killing becomes even more important when you consider that so many Steel types can easily get out of Magnet Pull. Forretress has Volt Change, Scizor has U-Turn (if it decides not to just use Superpower), Jirachi is commonly seen with U-Turn (although some people will just choose to spam Iron Heads. Not like Probopass can KO it easily. Can't risk it against Magnezone due to a x4 resist), and Skarmory is often seen with a Shed Shell. So Magnezone's ability to perform well against Pokemon not of the Steel type is very helpful.
With the reintroduction of Petaya Berry, we can expect some more Empoleon usage. Probopass can't take down the guy, Magnezone can. Small bonus for Magnezone, but can be helpful.
The only Steel type Probopass can reliably kill in OU is Ferrothorn. And it doesn't make much use of that ability, doing nothing while Magnezone will do the same thing in taking down Ferrothorn (while taking much less damage due to resisting Ferrothorn's attacks), set up a Sub, then get to +6 with Charge Beam, maybe even get up a Magnet Rise if you roll that way.
You also have Magnezone being able to fit in on Rain teams, which is cool since Rain is now the most common weather in OU. STAB Thunder is nothing to laugh at, nor is the ability to take down Ferrothorn, a common threat to Rain teams. And a lack of Fire weakness with Rain is also cool.
Probopass does get the Special Defense Sand boost, but do you really want to use Probopass on your Sand team in lieu of something far more helpful (such as a Terrakion or even the rare Regirock)?
There's a huge reason why Magnezone is in OU and Probopass is NU, with not even a mention being made for it in a tier higher than RU for the Smogon competitive section.
Edit: Just had to add on your stat calcs.
Lucario, assuming he's Jolly nature: 52SpAtk Probopass (Neutral) Hidden Power (Fire) vs 4HP/0SpDef Lucario (Neutral): 40% - 48% (114 - 136 HP). Guaranteed 3HKO.
Bronzong, assuming he's Relaxed Nature: 52SpAtk Probopass (Neutral) Hidden Power (Fire) vs 252HP/252SpDef Leftovers Heatproof Bronzong (+SpDef): 8% - 10% (28 - 34 HP). Guaranteed 13HKO.
Aggron, assuming he's Adamant or Jolly nature: 52SpAtk Probopass (Neutral) Hidden Power (Fire) vs 4HP/0SpDef Sturdy Aggron (Neutral): 22% - 26% (64 - 76 HP). Guaranteed 5HKO.
Scizor, assuming he's Jolly or Adamant nature: 52SpAtk Probopass (Neutral) Hidden Power (Fire) vs 4HP/0SpDef Light Metal Scizor (Neutral): 72% - 86% (204 - 244 HP). Guaranteed 2HKO.
Metagross, assuming he's Jolly or Adamant nature: 52SpAtk Probopass (Neutral) Hidden Power (Fire) vs 4HP/0SpDef Metagross (Neutral): 30% - 36% (92 - 110 HP). Guaranteed 4HKO.
And at last, Magnezone, the one who you think is weaker than Probopass, assuming he's Modest or Timid: 52SpAtk Probopass (Neutral) Hidden Power (Fire) vs 4HP/0SpDef Sturdy Magnezone (Neutral): 32% - 39% (92 - 110 HP). Guaranteed 4HKO.
Factor in Sunny Day to these calcs (something, I would like to repeat, that Magnezone can also do) and you still only manage to KO Scizor, who still has Superpower and U-Turn. So setting up a Sunny Day and KOing it is incredibly unlikely.
