I have a 380 that I love. After reading most of the comments on the 388, including Ale's awesome review, I was convinced to get one. I have a 312 which I love as well, and the 388 sounded like a great blend of that and my 380. I went to my AD and tried on the 388 ... surprisingly, I just wasn't moved. The 380 has a charm all its own - it's slimmer than the 388, and surprisingly, the painted dial with bigger numbers, etc., looked much better to me than the 388. I gotta say, I was surprised. Even though the 388 may be a better rep, automatic, and have a date, I found the 380 to be the more pleasing Radiomir. I would suggest test driving both of them in person if you can before choosing as they have different charms.
While I was there, I tried on the 389 too. In spite of its daunting 47mm size and barely hanging onto 1st class rep status, that's the one I'm ordering next. It's just beautiful.
Good luck with your decision. I'm going to upgrade the original rep strap on my 380 and be even more pleased, me thinks!