I've done a number of Road Scholar programs (13) and recommend you stay with the ones specifically marked Small Group. Nothing wrong with a larger group but it just takes so much longer for people for comfort stops/food, etc. Small groups are capped at about 24 which works for me. I've done 5 US based programs, 1 hiking, 2 birding and 2 genealogy. The others have been international.
The Road Scholar forum discussion pages are dreadful and pretty much defunct. I'm on 2 Road Scholar FB pages, Women of Road Scholar and Friends of Road Scholar. Both of those will likely have folks who've been on whatever tour you are considering.
I'm not familiar with Far Horizons...just looked at their website and whew, they are a lot more expensive. I'll be interested to see comments from others who might have traveled with them. And on top of the price they want a $150 for donation to a project. Yikes.