Re: If you had to choose one... and why?
Overall, I prefer the 4.0's for ease of use (just slap it on and you're done). If all babe did was pee, flips would be perfect, but IMO 4.0's are WAY better when it comes to cleaning up after poo.
Having said that, I have a mini-stash of both and use them at the same time. I use the flips for at home...the 4.0's for night-time, errands, and when I'm in a hurry at home. I use flats for both (padfolded) to make everything easier.