I think it depends on whether you're shooting in a controlled environment (like a studio setup) or "in the wild". TTL flash, especially the old kind which reads light bouncing from the film rather than the modern version which uses a preflash, can be very fast and accurate — as actually can the older "auto thyristor"¹ models. But, it's also going to be unpredictable.

If you have a controlled environment where ambient lighting, subject distance, and your position aren't changing quickly, manual flash with metering is probably better — true on digital as well. This lets you balance the light between different sources (including multiple flashes, ideally).

You can use a stand-alone light meter, but another approach is to take test images with a digital camera and then replicate the settings on your film camera. (Obviously, keep the ISO on manual to match the film; set the shutter on both to the sync speed — then you just have the aperture to match, once you have flash power how you like it.) I know many portrait photographers in the film days used polaroids for this purpose (possibly swapping in a polariod film back for a medium format camera — now, digital can do the same thing for the modern film photographer with many advantages.

1. Not actually a thyristor in many units but still usually called that by photographers