I have done trips with a friend and former co-worker, two ACA trips (one van support, one guided but no other support), and some longer solo trips. At this time I am thinking about two trips, one with a friend I have toured with before and the other would be solo, each would be very different trips.
You did not say if there is a financial reason to do it one way or the other. Therefore I assume that budget is not part of the decision.
You also did not say if you have schedule issues, such as you could have the summer off but have to be at work at a certain day, such as some teachers. Or perhaps you are retired and can be open ended on final date.
You were silent on your past bike touring background. And you were silent on your past camping background.
And you were silent on if there is a particular reason to not be part of a group, such as you may have dietary issues. For example, the last time I did a week long ACA trip, I could not eat many of the meals, and it took me almost a month after being home to get my blood sugar under control again because some meals were pure carbs. If I tried to do a multi-month ACA trip, I would probably be in the hospital half way through it.
So, given all these unknowns, these are my generic thoughts.
ACA group trip, you should have the knowledge to fix basic stuff on your bike, you do not need to bring cooking gear but you will have to carry your share of the group cooking gear and supplies. There probably is someone in the group that is knowledgeable about basic bike repairs if something serious breaks, they likely will not have the tools to fix something serious but can diagnose it. The guide may have info on where the next bike shop is. One of the group trips I did with ACA, one person had a bottom bracket bearing go, he was lucky and the bike was new enough that he had warranty support. Another guy in that group had a front derailleur break, either the spring or spring stop I do not recall which, I managed to make it functional for the rest of the trip with a piece of bunge cord. If you have a serious mechanical on a solo trip, you are on your own, but you might get lucky and get help from someone in the campground that is also following the same route.
Solo trip, my last tour I saw five consecutive days of rain in the forecast, so I chucked my planned route and went to a small city that had a hostel and some sightseeing opportunities, stayed there for three nights in the middle of the six days of rain (forecast was five but it became six days) and did some sightseeing, some beer drinking, and hanging out with others from the hostel. If I had to pedal for six consecutive days to meet schedule in the rain with a bunch of other miserable cyclists that were doing the same, camping most or all of the nights in campsites full of puddles, that would be something that I would remember unfavorably for a long time. If you were in that situation, what would you have done, would you feel comfortable going off route and doing something completely different than planned? These are things to think about.
And, if you get sick for a couple days where you can't travel? I have been fortunate, that has never happened to me. But if that did happen to you, what would you do and what would the rest of the group that is committed to make schedule do if you were not solo? A friend of mine has gone to the hospital on two separate van supported trips after a crash, in both cases there was a guide and van available, I am glad that never happened to me.
Last point. I have traveled by air, in which case I knew the dates that my trip would start and end. On those trips, the last third or fourth of my trip is devoted to making sure that I get to the end point on time, perhaps with some contingency. I have also traveled by train at the end of a trip to get home, train tickets can be bought on short notice (few days) that are not that much more expensive than tickets bought weeks or months in advance. Thus, if you can travel home by train, you have a LOT of flexibility for how much distance you average every day. I traveled for a couple days with a gal that was going half way across Canada before she flew back to Europe, in her case her bailout plan to make it to the airport was to rent a car if she was behind schedule to finish her trip.
ACA trips, they set a schedule, months in advance they can say where the group will be on a certain date, weather is not part of the consideration, you go when and where told. Solo, you decide if you want to travel that day, how far, and what destination, and what route, and part way through if you change your mind you can, with complete flexibility on route, and if you run out of water in a very dry place you have only yourself to blame, while wondering where you will find the next grocery store or fuel for your camp stove.
Read the last paragraph above one more time. Close your eyes and think about that for five minutes. Did that tell you how to decide?
I did not tell you what to do but I did tell you what I would be thinking about if I was making the decision you are making.
A friend of mine has done two cross country trips with ACA that had a guide but no support. He did one Canada to Mexico van supported trip with ACA. And several other one week long trips where food, lodging and guides were provided in USA, Europe and in Asia. But, I do not think he has ever done a trip longer than one week solo where he had to make the decisions every day. His personality is not the solo touring kind of personality needed for that.
Good luck, and I hope you have a great trip.