In Episode II we see the early plans for the Death Star (initially developed by the Separatist Geonosians, led by Poggle the Lesser) at the Battle of Geonosis:
The Battle of Geonosis was the first battle of the Clone Wars, and prior to that battle the Separatist Council was not even aware of the existence of the clone army (only the Sith knew, since they were playing both sides). Indeed, the Separatists were confident that their combined droid armies would give them military superiority over the Republic:
With these new battle droids we've built for you, you'll have the finest army in the galaxy.
Wat Tambor, Star Wars: Episode II Attack of the Clones
If the Separatists (excluding the Sith) were so confident in their droid armies, why did they also develop plans for the Death Star? A planet-killing superweapon seems like massive overkill from the non-Sith Separatists' perspective. And considering that it took decades for the Empire to build the Death Star, how could the Separatists have thought that it'd be useful before the war was over? With the Republic apparently defenseless the Separatists must have thoughthought that the war with the Republic would have been a short and easy victory for them.
Of course, the Sith wanted to develop the Death Star since they were controlling both sides of the war and wanted to use it when they conquered the galaxy. But the other Separatists did not know of these plans, so why would the other Separatist leaders (who all seemed to be aware of the Death Star plans) want to develop the Death Star?