Emily,
Xpages is the Domino development platform of the future. You could stay with Notes Client apps, or even Classic Domino Web apps but they're not exactly adding any new tools to those design patterns. They are for XPages. XPages is also standards based and compitable with common source control platforms.
I assume you are currently building apps with Classic Domino? If so then what's the effort involved in doing just about anything? How much HTML code to you need to hide in view columns? what's the effort in adding a pager to a view? I honestly don't know the answers to those because I was a client dev for the most part before XPages comes along. But I think Pagers for instance was a significant effort. The two biggest features to me are: Repeat Controls and Scoped Variables. how nice is it to have an in memory variable that can span pages if you want? No cookies needed.
If you are a client dev then there's no question. XPages and it's use of standard html, css, javascript, etc... allows you do have whatever UI you want. Seperating the document from the form is a big plus as well. I think Scott Good once told me when he looked at his first real XPages demo at Lotusphere. "We could do everything they just did currently. But it would take a heck of a lot longer to do it."
I don't believe you can easily replicate repeat controls and scoped variables in classic domino. Certainly not Themes which if more difficult to get into but extremely powerful. Heck look at the 100+ controls in the ext. library, the tons of snippets at XSnippets, and the other custom controls that the community has contributed free of charge. Try and replicate those in Classic domino. Some just aren't going to happen. The ones that might be possibly would take a long time.
Check out my 2 jumpstart videos on
xpages.tv Those were done a while ago but were intended to wet the appetite and show you how to do some things. Many of the other shorter videos probably would be difficult to reproduce in classic domino in the time it took me to do them in XPages. XPages is not perfect, there's work involved and a learning curve, but it's by far better then anything else that we've been doing.
If you need more advice just reach out to me.