The term stable has a couple of subtuly different meanings. There is the "doesn't crash very often" meaning and the "what worked today will work in the future" meaning.
The only reasonable way to get security updates for debian testing/unstable is to keep rolling forward with them. That means you are close to the bleeding edge but it also means you are building on something that is distaincly a moving target and that can and will break your stuff. The same applies to the development releases of ubuntu.
At the other end of the scale you have Debian stable and Ubuntu LTS. Lots of stability but also old software. The release cycles are not synced so sometimes it makes sense to choose between them based on which of the two had a release more recently.
What Ubuntu offers that Debian doesn't is the 6-monthly releases, these offer something of a middle ground on the stability/up to dateness scale.