Late to the party it seems…
Play nice guys.
On the flash: If the distance from the flash to the subject is (more or less) constant, manual will be consistent. Also: Manual flash can be triggered over long distance by a relatively cheap radiotrigger.
ETTL flash needs line of sight, not too much interference and a master flash, or (the only real option in cases like this) a more expensive ETTL radio trigger.
I use my on camera flash set to ETTL mostly (wildly varying distance to subject), but sometimes set it to manual if ETTL screws up.
I use off camera flash on manual, always.
When using a manual flash on a not easy to reach place, you can (within limits) control flash exposure by adjusting ISO or aperture. You can adjust ambient-flash ratio by varying the shutterspeed.
Audience shot, ETTL flash (zoomed in a bit) on camera, manual flash via radio trigger on the camera-left side of balcony I'm under.
ETTL flash on camera, manual flash behind them, on the balcony again.
ETTL flash on camera, manual flash just hidden behind the laptop:
Last shot is the only one where using an ETTL optical slave would have been an option. (been there, done that)
I hardly ever use flash when shooting bands, since mostly lighting is adequate.