Even if a SaaS solution may seem like a lesser version of its desktop counterpart, the former has many advantages over the latter that may justify its development. A desktop application lives in the computer and, if we update the operating system there may be incompatibility issues; if we buy a new computer we might have to reinstall the software, also we cannot use it on mobile devices. A SaaS app lives in the cloud and runs in the browser, and if we start working on a desktop computer, we can resume our job on mobile or using a different computer: office, home, or on the road, the software always follows us. Another point for SaaS is that there are a lot of developers specialized in web technologies, and they can work in an existing infrastructure; this keeps costs for developing SaaS solutions lower.
The final choice depends on what our final goals are, and these have to be clearly defined: trying to build CAD software that must challenge the best-in-class… well, it can be an overwhelming challenge and an SDK surely offers more possibilities than web programming languages. Building an app as a SaaS solution must have clear advantages over desktop. It may be a very specialized app that offers the same experience on every device or it could be just a reduced version of a desktop application, less powerful but easier to use and, thanks to reduced development, maintenance and distribution costs, much cheaper (or even free).