If you’re creating software without a mobile consideration—such as for intranet portals or content-heavy, browser-based applications—Flash would be the best technology for the user interface. If accessibility from mobile devices is a requirement, HTML5 might be the better choice.

These are the observations of Anthony Franco, president and cofounder of the consulting company Effective UI, who said it’s becoming more critical than ever for people executing on the application’s front-end design to understand the needs of the user.

However, the conversation is made more complex by Apple’s refusal to allow the Flash runtime onto its iPhone and iPad devices. In an open letter posted to the Apple website in April, Jobs explained that Flash technology is not only proprietary, but claimed that it’s also old technology that uses too many resources and crashes too often. Adobe, of course, rebutted that Apple simply wants to keep tight control over everything that runs on its company’s devices.

The irony, according to UI designer Jeff Gothelf, director of user experience at TheLadders.com, is that Jobs is becoming a proponent of a very open standard HTML, while everything else Apple does is closed and locked down.

From Franco’s seat, the war between the technology giants “is not helpful for developers. The conversation has not progressed beyond ‘Apple sucks’ or ‘Adobe sucks.’ From the user experience, it’s annoying that I can’t see all content on the Internet on my iPad. That makes it a poorer user experience.”

Because of the rift, Franco said, developers have two choices: “They can either build [the application] twice, or build it less”—meaning the desktop- or browser-based application might lose functionality if it uses HTML5, which lacks the richness of Flash.

Most large enterprises are making the decision to build the application twice, said Franco. “It’s a philosophical question. The Holy Grail has been to write once and deploy anywhere. But that doesn’t address user adoption. The experience has to be built for the device people most want to use.”

Gothelf said, “You can’t deny the traffic being driven by those devices. It forces you to rethink your application. If HTML5 can’t handle it, then from a design standpoint, you might want to simplify the experience.”

But writing the application twice might be the best choice for organizations that must have the best experience possible for the device or computer upon which the application is run. “A touch interface versus point-and-click, and a significantly smaller screen resolution, these are significantly different experiences,” said Gothelf. HTML5 might lack the power and level of flexibility of Flash, he said, but Adobe needs to streamline its code to make Flash easier to work with.

Franco said the challenge here for application architects, designers and developers is making the decision as to which way to go. “It’s a question of which platform to build on. Is it maintainable? Scalable? Will the platform be around two, three, five years from now?”

It all begins with user research and gathering good user data before design, interaction or technological decisions are made. “Bluntly, that’s a step that’s often missed,” Franco claimed. “If the CEO says we need an iPhone app, we can validate that or not with user research.”

Gothelf sees Flash continuing to reign supreme for online advertising and banners, but for page-level experiences and transitions, “there’s no reason to force Flash.”

He added that now there is a skills gap, as most people working on Web applications today are comfortable with Flash, and HTML5 is still so new. “When people get up to speed [with HTML5], you’ll see it become much more pervasive.”