Another proverbial quip:
"He travels fastest who travels alone."
There is certainly a lot of truth to this proverb. Additional people mean additional idiosyncracies and additional agenda, all of which conspire to slow down the journey. I find that it is difficult enough to travel as a husband and wife, since this always requires negotiation about where to go, when to go, and what to see. Lee and I have reached a stage in our traveling where we can almost anticipate what the other’s needs are, and we are certainly considerate of each other’s needs and desires, so we have adapted. But, on other trips, we have included other couples and this can become extremely stressful. Now, the itinerary has to accomodate four people, each of whom brings their own "baggage." I am reluctant to do this ever again!
From a different perspective, traveling alone is really not much fun. Sure, you can get to everything you want, and spend as much time as you want, but who can you share it with? A large amount of the joy associated with traveling involves dialogue, comparison, and perspective, and these are greatly enriched with more than one person.
Therefore, my preference is to travel with small groups, ideally two people, who are very close and know one another very well.