I had a very disappointing experience at Windsor last month. Way too crowded. Waiting time in queue/line for Queen Mary's Dollhouse was 45 minutes, standing, weather elements - light rain.
This was not my first Windsor Castle visit. I took a first-timer who was overwhelmed by the numbers and chaos of several large school groups. At times, the noise was such that the audio guide was hard to hear. Expensive, unsatisfying morning all 'round. I would hope this experience was not the norm, but if you chose to go, be prepared.
Buckingham Palace two years ago was a contrast in decorum, crowd control (there were no crowds) and enjoyment.
Very different sites, as noted above. Rather like apples and oranges. Both should be outstanding. I hesitate with recommending Windsor due to my recent visit. I'm not a crowd hater - I can handle Versailles Hall of Mirrors quite well knowing how to approach it. Windsor rivaled Versailles chaos on my recent visit, but given the price, limited timed entry tickets, degree of security and scrutiny and guard observation in the rooms, I would have expected better numbers and crowd management.