Having a knowledgeable guide could be a big plus, but otherwise I'd say visit on your own, preferably at a time when it won't be as crowded as it can get. You can use a guidebook for detail, plus there are very knowledgeable and helpful docents in each room. I asked several of them very specific questions about history, furniture etc. and got good answers. They have reference binders covering each piece of furniture, vase, lamp etc. I spent most of an afternoon in the castle and chapel, and left feeling very satisfied with the visit. One thing you can't do in the castle is backtrack -- it's a one-way route through the different rooms, but you can stay as long as you want in each. There was more freedom of movement in the chapel.
I hope you know that Buckingham Palace is also open for visits in August and September. You have to sign up for a specific entry time. As at Windsor, there was plenty of time to appreciate the rooms, excellent docents everywhere, etc. I think there are also evening-with-drink tours there for higher prices. I visited these two places a couple of weeks apart, not sure I'd recommend them on the same or consecutive days lest you OD on Royal Spendour! ;-)
