Hi Gin
It's difficult to say which is the best time to go. The mean temperature in that part of the US in June is about 75-80 degrees farenheit (24-27 degrees celsius), so if you don't like hot weather you might prefer the March or September departures.
If you have a look at the tour managers page of this website, you'll see a message from Daniel Adams (12/1/09). He is leading the tour in late March and will know a lot more about the area than me! So please feel free to ask him for advice too.
You will certainly meet plenty of friendly people on one of our tours. All of our customers are single travellers, so everyone is keen to meet new people and enjoy the holiday as a group.
As for your other question, around 20 people have booked the March departure so far. Around half that number have booked the June date, which is still quite high considering that the departure is more than five months away. So I wouldn't say it was less popular - just happening later in the year!
I hope this information helps.
Kind regards
Olly, your editor