my vote is database concepts. With the rise of PaaS options out there the need to be able to build distributed systems yourself is lessening (unless it's referr

my vote is database concepts. With the rise of PaaS options out there the need to be able to build distributed systems yourself is lessening (unless it's referring to decoupled systems, such as standalone API endpoints) and falls more into the sys-admin domain. However database concepts will directly relate to your web dev career and will help improve your architecture design and general programming skills as well.