ok, here you go: i have compared both quite intensively, and indeed i bought the new one too. one of them will be a present for my mother.
the biggest improvement is of course the new cover. it doesn't need such a big footprint anymore, and even when open it sticks to the bottom of the speaker and won't dangle around.
there is definitely a difference in sound between both, but it is not that easy to explain.
soundwise it depends a lot how to judge the old soundlink, especially at which volume setting, it sounds considerably different dependent on the volume you set it.
usually i always turned up the volume on my player to the max and then set the volume on the soundlink to an adequate level. but meanwhile i noticed that the old soundlink sounds considerably more balanced if you set its own volume to the max, and then turn the volume on your player down to the right level. as the old soundlink emphasizes both mids and treble at its highest setting, this trick will result in a more natural sound with a better controlled bass at lower levels, otherwise the bass tends to drown everything else.
interestingly this trick doesn't seem to have any effect on the new version. it sounds equal independent of the volume setting.
if the trick is not applied on the old soundlink it will sound boomy and muffled compared to the new one. but if you apply this volume trick the differences are not that huge anymore. still the new one has better defined mids, vocals sound more natural and the bass is tuned deeper, which doesn't have that boomy character of the old one. unfortunately with the new one the upper treble is more reduced than before. i can hear some hihats and tambourine sounds on the old one, that are hardly noticable on the new one. i still think that the new one has the more balanced sound, but i would have preferred the treble clearer.
i also have the impression as if the new one doesn't sound that harsh and strained at maximum volume in return the new one seems to have a slightly lower maximum volume than the old one, this is probably also due to the more balanced sound while the old one pushes bass even stronger to the background, which is again an advantage at lower volume.
it's a hard pick, if i got the old one for a resonably low price i would definitely go for the old one and then maybe buy an additional new cover and keep the original one as replacement. still the new one sounds better with vocals, but you could tweak any of them to your liking with some EQ. unfortunately the EQs from apple are not really usable, therefore i prefer if the sound is perfect from scratch.
also note that i really compared both side by side and i am quite picky soundwise. many people might even not notice any big difference between both.
i hope this helps, although i have the impression as if it won't
