I'd guess that 90%+ of people would have difficulty telling the difference between a decently encoded mp3 and a flac file in an a/b test (although I'm not including the 128k mp3 in that

, you're getting a little iffy there). That 90%+ probably includes a fair number of those that insist they can hear the difference

. I'm not saying everyone, because I know there are indeed people that can notice.
Personally, I'd rather just use the smartphone for me, because I'm going to be carrying it anyways, and likely am going to be charging it every night regardless, so it's not really costing me anything to do that. It's just not worth the extra equipment to carry around for a separate mp3 player. I've frequently got enough other electronics I'm carrying around as it is.