I love Steamboy, but it's so clearly a reworking and expansion of Miyazaki's Laputa: Castle in the Sky, from the boy of humble birth and girl of high birth to the giant mobile castle to the little object of great power. There's even a particularly pointed steal from Laputa--the moment when the boy at the lip of a precipice leaps up for joy with the girl in her arms, and nearly falls over (a reprise of a similar scene when boy and girl first land at Laputa).
I love the details in Steamboy--the way they worked out that anything is possible so long as a compact and lightweight power source exists (hence the 'steam balls'); the way the machinery look so 19th century, and familiar because they are based on countless old published sketches, but are introduced with such buildup and drama that when you see them in action you gape in wonder anyway; the idea that low-temperature, high-pressure steam isn't hot but actually sucks heat from its surroundings (enables Otomo to create a few vivid images, including a windswept London and a climactic explosion that avoids the "mushroom cloud" shape that has become such a cliche in most anime).
That said, I still think Miyazaki's Laputa is the superior film, and not just because it was first. It's perfectly paced, where Otomo's tends to drag in the third act; its characters are far more vivid overall, especially the boy hero (he's given gestures and moments--the pigeons he serenades with a trumpet each morning, the handful of gold coins he flings to the ground--that stay in mind, even years after I first saw the picture). The girl Sheeta is perhaps less vibrant than Otomo's hilariously headstrong Scarlett O'Hara (perhaps the best--or at least most fun--single character he's ever invented), but Sheeta's helplessness and passivity is crucial, in that it makes the moment when she finally makes a stand all the more dramatic. Not to mention the pirates in Laputa--and the villain, the rare decadent sadist in Miyazaki's films--have it over the entire supporting cast of Otomo's picture.
Themewise, Steamboy is sophisticated enough to make the distinction as to what technology should serve--man's desire for power and greatness or his need to help his fellow men. Laputa has an ostensibly hoary theme--technology without wisdom is destructive--but acquires an interesting sidelight if you consider Miyazaki's own character: the girl's final speech, that these flying castles (symbols of high ambition) need the ground (symbol of humility and level-headedness), stands out as a repudiation of Miyazaki's love for flight and fantasy. In effect he's reminding himself to keep his feet on the ground, even while providing us with one of the most gorgeous aerial adventures ever created.
Having just used Laputa to thoroughly bash Steamboy in the head, I do have to say I can't understand critics who prefer Otomo's earlier Akira to this one. Granted that the former has a more ambitious scope, and if it seems cliched today that's because it invented many of the cliches, I just don't get it when, say, Film Threat claims that the runt-turned-superman in Akira is interesting (the idea is interesting, if not new, but what Otomo actually does with him isn't), or that the earlier film shows how anime can be emotionally involving. I was involved with Akira, but never emotionally; I thought it was a prime example of stupendous eye candy and weak characterization (think it's true of both anime and manga). Wouldn't put Steamboy in the same league as Miyazaki's films, much less Laputa (one of his best), but I'd definitely consider it Otomo's best to date.