OK.... here we go...
The ending is the pretty much the same except there is an entire episode dedicated to a trial. He is framed for the murder of the blind girl, as the accordian man kills himself (jumping off a bridge or something...)
Arthur sort of did ruin Eileen's life, she did become and prostitute and get an abortion, but I think that she loved him anyway, as she pretty much stuck by him till the end.
He does give up everything to be with her, putting off a whole string of events leading to his arrest and trial. Joan's character isn't as severse as the US version while talking to the inspector ("Cut off his thing!" isn't present in the original).
As a whole, I have to say the original mini-series is better. The script of the US version is totally watered down, keeping in the memorable numbers ("Love is Good for Anything That Ails You",) and adding in a few new ones ("Ain't Misbehavin'", for example,) though the numbers weren't as vibrant. I did appreciate the production values of the remake, and the musical scenes where exquisite, most memorable, to me, being "Pennies from Heaven," in my opinion one of the best motion picture dance sequences of all time. In this way, the remake hails greatly over the original. But other than that, the acting isn't as good, although it was still well performed. And again, the script (and the whole story, for that matter) is totally watered down.
(minor spoiler warning)
I'm watching the miniseries now. One indication of the difference between the film and the miniseries is the scene where Eileen is fired by the school headmaster for being pregnant. In the movie, if I remember right, Eileen figures out that her doctor is also a school board member and that her secret's out; the headmaster throws her out. The whole scene takes 90 seconds, if that. In the miniseries, this whole scene takes up the majority of an episode in itself -- perhaps twenty minutes: in which the headmaster approaches Eileen, and the both of them dance discursively, both metaphorically and literally, around the topic of her situation and the necessity of her termination at the school; the headmaster remembers her as his own student, and his whole history at the school, as well as his relationship with her, blossoms in subtext. It's very moving, and a fuller, richer entertainment than the comparatively threadbare film.
Another important difference between the miniseries and the film is the acting styles. The American actors are uniformly surly -- Steve Martin struck me as glaring throughout his entire performance. Jessica Harper, though a little more modulated, was still a one-note misogynistic shrew. Bernadette Peters was affecting but slight, and somewhat unconvincing. Gemma Craven and Cheryl Campbell outshine their American counterparts like supernovas destroy flashlights. And Bob Hoskins' performance is sublime.
I should warn you -- he's a Fourierist.
I agree that the original is much richer, though the remake has some nicely done numbers (notably: Love is Good for Anything that Ails You, It's the Girl, It's a Sin to Tell a Lie (short, but moving), Let's Misbehave).
Some of the ambiguity is lost too.
For example, in the original, there is an element of doubt about who killed the blind girl. (The accordion man, a disturbed character, might have found the corpse and imagined that he killed her.) The remake removes all doubt.