FWIW, we're going through the same deliberation process. DW wants to go with a caravan for the "security"; I'd be fine going solo. If we do the caravan route, though, we'd probably go with the Adventure Caravans 59-day tour:
http://adventurecaravans.com/tour/alaska-the-canadian-rockies/. They offer a fair amount of flexibility on the daily schedules, allow one to join or bail out of the trip at any time (with corresponding adjustments in the price), and have said I can change the duration of the stops to meet our interests (e.g., I think I'd like to spend less time in Fairbanks, get to Denali ahead of the group, and wait for them to join us later). When I roughed out a solo Alaska trip and compared it to the Adventure Caravans version, the two were almost identical.
I'm sure you've seen it, but Joe & Vicki Kieva have a book on RV'ing in Alaska, and the chapter on "Solo or Caravan" is available online:
http://rvknowhow.blogspot.com/2011/01/alaska-solo-or-caravan.html.
We haven't made a decision yet--we're going to talk to the Adventure Caravans people at the FMCA rally in Perry in March. I'll be smarter in a couple months.
Keep us posted on what you decide.