The Sterling DCDC B2B functionality is to front-end **any** 12VDC source giving adjustable charge profile to care for an expensive bank of a chemistry with precise needs. Nothing to do with solar controllers, certainly not an either-or choice.
For solar, my preference is to buy separate top-notch solar controllers sized for a particular setup's specific needs.
I like Victron's SmartSolar MPPT line, those bundled with DCDC units are not at all on par.
CTEK and Redarc make fine durable products, the DCDC component is lower cost than the bigger Sterling BBs but have very limited current capacity and they are not customizable on setpoints
for someone with very simple needs, keeping a battery topped up while the van's stored in the sun, of course fine
but neither bundled SC is user customizable on setpoints, do not allow high-voltage panels, have very limited current capacity and lack logging / reporting or integration with BMs.