A lot of what you ask and the question of is one better than the other it's completely subjective. What one person claims looks nice can look completely horrible to someone else.
If you constantly pause MAME and want to flip through different windows then -video d3d HLSL is easier to do that from my experiences.
There's also the presets in the ini folder (Gameboy, GBA, Vector) that looks really nice with -video d3d hlsl
I myself leave the backend to auto and assumes it picks the best one :P
The pro to using BGFX with screen chains are, you can flip through different configurations during run-time without stopping the emulation or closing MAME.
I'll attach the hlsl.json and raster.ini that I use and you can decide if you like how they look.
hlsl.json use with BGFX
raster.ini use with -video d3d -hlsl_enable and put this file in location MAME\ini\presets