When should I use HDL or visual tools?
If you want to build with 7400 series parts, then you may be better off directly designing with 7400 series parts and drawing up a schematic.  However, it is possible to design in HDL and synthesize for 7400 series logic.  The open source synthesis software yosys can actually target 7400 series logic for synthesis.  It may end up being a gigantic pain to wire up and debug for a complex design, but it is possible.  

I think it really depends on what it is that you want to learn.  If you want to learn modern hardware design, then out with the 7400 logic.  Get a reasonable FPGA dev board and work in Verilog or VHDL.  And FYI, modern hardware design is not done at the gate level, it's generally done at the behavioral level with conditionals, register transfer operations, state machines, etc.  The synthesis software then turns that high-level design into optimized logic that that targets your target device/process.  If you want to build something out of 7400 series logic 'just because', then work at a schematic level and design directly with 7400 series parts.  You can certainly write gate-level HDL (for most things, anyway) to validate that your design will work.