I own them both and I like the Ami more. It feels more comfy inside me and it's easier to just put the Ami 2 in and go. Ami is also less work to clean. The only downsides to the Ami that I have found are that Ami 1 is too big for me (and a majority of others and maybe you too) and it doesn't come with a storage bag like the mini luna. But I do agree with married with children when they say that the Ami is much better quality. Lelo could have done better with this set and improved upon them rather than just making them smaller.
I'm okay with the mini luna but they are a bit too loud for me. So I really only wear them when I don't care what the public around me thinks/hears. Not all the time but sometimes I will move in just the right way to get a nice, very audible, rattle out of them. The string is also a huge downside...I don't think you could ever get it as clean as the balls and holder. Some times the rattle is very nice, though. When I feel it I feel a bit more naughty.
Ami 2 and the Mini Luna are basically the same size. Ami is a better value in that you get another exerciser to move up to when you want to. Ami 3 is very heavy compared to the other two and I can't wait to be able to use it.
I should really review them eventually *lazy*