I quite enjoy private tours. I like the opportunity to customize our itinerary and destinations, as well as schedule throughout the day. I like to be able to spend as much time as possible in places I really enjoy, and not feel rushed. But obviously different tour types can have their advantages and disadvantages.
A private tour (either just my spouse and myself, or another couple or small handful we're traveling with and know from home) has the advantages above, and I tend to make conversation enough with the guide and travel with people with similar interests there isn't too much quiet time. I'll ask questions, love getting to know the guide and hearing their stories and insight, getting information from their perspective, and generally just making conversation with them. A disadvantage can be the lack of outside perspectives - sometimes on a small group tour that includes people I don't know, someone else will ask a question or solicit information that I never thought of. It can help bring about fresh new info and opportunities to learn things, and see and do things. Is also a great way to meet new people and get to know fellow travelers!
Larger group tours also mean chances to meet new people and hear different info, but I've found in practice there's often been one or two individuals who treat the large group tour as their own private tour - asking a disproportionate number of questions, monopolizing a lot of the guide's time, and so on. I can't fault them for their inquisitiveness and eagerness to ask things, though. They're certainly getting the most out of their tour, and perhaps the rest of us are at fault for not speaking up more ourselves! Overall, though, I'm not a huge fan of large group tours or the big bus tours and such.
I've found a great guide can really make most any sized group worth being a part of, though - making sure there isn't too much quiet time on a private group (or even making sure their is, if that's what the travelers prefer), being attentive to everyone on the tour, making it a point to reach out to everyone and engage with everyone, and keep the tour focused while at the same time connecting to everyone and not letting it feel to rigid and formal.
Really, it's possible to try and make the most of whatever one's budget allows so long as expectations are realistic!