I think headsets are more safe for cockpit-ground communication than signals. I've done both of them hundreds if not thousands of times. If something goes wrong during engine start it's faster to shout stop over the phone, than first trying to get attention to a crew watching engine rpm's-egt's and other red lines. I also find that crews nowadays are sometimes not used to handsignals anymore, last time I did the sign "chocks away", the f/o did react somewhat funny to me. Obviously he did know what I was signalling, he was just not used that we did it that way and it confused him. At night with ramp spot lights full on, it may also be difficult to see what the flight crew is signalling.
Headsets on the other hand are not of any use if english is in poor condition- am I glad I don't have to do startups of Tupolevs anymore...