Sorry for double posting, but here's another comparison.
Registeel as a defensive Steel-type vs. Bronzong
vs.
Registeel's Advantages
Greater Bulk: Registeel has better bulk on Bronzong on both sides, which means Registeel is sturdier than Bronzong, and hence can take more hits. It can as a result wall some things such as Raikou, Shaymin, and Roserade more effeciently than Bronzong, while still providing team support in Stealth Rock.
Thunder Wave: This move can be incredibly annoying; while they both invite dangerous Pokemon such as Darmanitan and Heracross in for absolutely free, Registeel can invite them and cripple the opponent with Thunder Wave, which is greatly helpful for much slower teams that would like the paralysis support. This can also be rage inducing, and if they get full paralysis then you can wear them down even more in the process.
Clear Body: This prevents anyone from lowering Registeel's stats, and as a result, it can keep its defenses rock solid 100% of the time. What does this mean, you say? This means Registeel can more efficiently wall Shaymin, as Seed Flare won't cut its hard Special Defense in half and can continue to endure repeated onslaughts from it. It also prevents stuff like Crunch from lowering its defense if you'd like to come in on those.
More resistances: This means Registeel can endure Megahorns, Shadow Balls, and Crunches better than Bronzong. This can allow Registeel some more switch-in opportunities.
Registeel's Disadvantages
Lack of offensive presence: While Bronzong has a decently powerful Gyro Ball to work with, it's still okay in terms of power. Registeel has to rely on Seismic Toss to deal legitimate damage, and this can often lead to Registeel being setup bait at times.
Lack of Levitate: This means that Registeel gets murdered by Ground-types such as Nidoking and Krookodile, while Bronzong is capable of taking anything from them. This is a great disadvantage that Registeel wishes it had. In addition, this means Registeel gets worn down more quickly, as Spikes damage will be hurting it multiple times.
Fighting-type weakness: Registeel has a massive Fighting-type weakness, which is a very bad thing in a tier packed to the brim with Fighting-types. Bronzong does not have this weakness, and can take some hits from Pokemon such as Scarf Mienshao decently.