ACT-R was the first big cognitive model and excels at modelling human behavioral data quite accurately.
Spaun, which is the world's largest functional brain model, took a lot of ACT-R's insights and applied them, most notably in it's model of the basal-ganglia-thalamus loop. More importantly, it did so in biologically plausible manner while ACT-R has been struggling recently to map it's functions onto brain areas and functionality.
That being said, ACT-R still seems to have an extremely active community and Spaun's developers are mostly restricted to the lab from which it originated. An obvious reason for not converting is if Spaun's range of cognitive functionality is missing some components of ACT-R.
What functionality of ACT-R isn't found in Spaun?