Yes, 15th and 16th will be a good option.
My opinion: Since this is very short meet (just one night), should we really need to bother about the sight seeing. If we go to Yercaud, the families will get disappointed, if you do not take them to all sight seeing places.
I feel that HH is calm place (as said by many) will allow us to get to know each other and have fun in one place, instead of travelling here and there. And of-course the families will also get company to play and talk. And moreover HH will be a new place for many, I believe.
This is just my opinion, I am okay for both the place.
@Sethu, please count me in 1+2.