I've had both, but not in the same color. I don't think the Miu Miu is too casual at all -- I actually think, since it's a little more structured and has the bows, that it's less casual than the city. I think the bow satchel a super-cute bag, but I didn't find it as easy to carry as a City. The City is really nice and light and flat and squishy, whereas the Miu Miu was a little more substantial and bulky -- if you're going to carry over your arm, then it's fine, but I like a shoulder bag. I think the Miu Miu is a little girlier (which for me is a good thing), and I agree that it will probably not be available for as many seasons as the City, which could be good or bad depending on whether you want to carry something that's always current. I also didn't like the black Miu Miu because the gold hardware and name plate was a bit too flashy for me, but it's pretty in the other colors; similarly, I wouldn't do a giant hardware City. It's a tough call, and really comes down to your day to day style and whether the bows will go or something a little plainer is better.
Edit: I just voted for the city because I don't like the gold hardware on the black Miu Miu. If you'd said nude or smoke grey Miu Miu, I might have gone the other way.
