Think the answer depends on your endurance factor.
I’m an early riser up and out a door before 6am. If you can get up without jet lag then go for the 8am slot. However if not early risers then the afternoon slot. Nothing would be worse than not being rested enough to enjoy the tour and your surroundings.
Strolling about London, Rome, Paris, Venice et al before or just after sunrise was an enjoyable way to experience these famous cities. Taking photos without humans getting in the way, joyful.
For me I like seeing the cities wake up, watching deliveries occurring , piazzas, streets, sidewalks being cleaned and all the workers who execute their jobs before dawn breaks.
It’s a frame of mind….all the worlds a stage, so I enjoy the actors. I also throughly enjoy NOT dealing with crowds. Honestly by walking and exploring so easily one discovers so much. Pocket parks, cafes, church yards, quiet beaches, unobstructed views and the best part, no screaming babies or children, piped music, or horns honking. Occasionally garbage trucks, street sweepers. Often joggers and dog walkers. Not a night person. I like my 8 hours and adore watching sunrises.
Be realistic and honest with yourselves!