Comparing liquid and solid stages, it seems like significant reason to use a solid booster is to maintain proficiency in building big stages which could be used in military.
For space launches it seems to me liquid stages have overall edge in usage.
>to maintain proficiency in building big stages which could be used in military
I guess the grand prize here would be to be able to build a smaller, but equally potent version of the Trident 2 [1] missile. This would allow the Navy to downsize the new class of strategic nuclear subs (Columbia [2]) while keeping their deterrence capability the same. Since the lifetime procurement cost of the 12 projected Columbia class submarines is about $350 BN, this downsizing could be quite profitable.
For space launches it seems to me liquid stages have overall edge in usage.