When all is said and done, aesthetics definitely matter the most to me. When building any ship that I intend to use much over an extended period of time, I typically come up with a vague idea of how I want it to look, build a frame/skeleton for it, skin the skeleton and finish some basic outer details and then I do the internals, weapons, etc, constructing a basic interior as I go.
For me, while I place some importance on functionality, if a ship that I build, excluding basic starting ships on servers and such, is not aesthetically pleasing to me, I don't fly it. Period. I also place a strong priority on having a bare minimum of exposed functional blocks for a given design, and further try hard to use what I call, "functional aesthetics", where "engines" on the outside of a ship are actually crammed full of thrusters on the inside, for example.
Once a ship is fully functional, I then add any extra external detailing that I want, finish out the interior, and make final tweaks to its systems.