In general for weldments I use hot-rolled because it's cheaper and more available. Once it is primed and painted it looks fine for most mechanical applications.
I use cold-rolled for general mechanical pieces and some welded parts when I want it to have a decent surface finish (like mounting faces, sliding surfaces, shafts, etc.) without having to machine the whole face. The surface of cold-rolled cleans up nicely with just emery cloth or scotchbrite (or nothing really), whereas the hot-rolled has a rough dark gray crust that requires machining or quite a bit or grinding/sanding to "clean up". The cold-rolled also has pretty good tolerances of flatness, thickness, width, etc. so it can often be used as is without machining even on parts that need some reasonable precision.
It is tough to machine the whole face of cold-rolled because the residual stresses for the cold rolling will cause the piece to warp when you machine just one side of it. Machinists usually have to machine both sides (maybe more than once) to get a piece straight/flat again. If something is going to get a lot of surface machining (not just boring, drilling, and tapping) then start with hot-rolled.
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