I believe it's really a personal preference thing and whatever you're accostomed to and comfortable with, will likely work fine. The business about delayed, or one handed signals, is really much ado about nothing.
For what it's worth, a number of years ago my finger whistle broke off at the finger band and the only replacement I could get quickly, locally was from a hockey supply outlet. The Hockey official's finger whistle is larger than what was available for football, but the sound is unbelievable. Crowd noise and wind doesn't bother it at all.