[Spoiler Warning]
Advice #1: Do NOT watch the movie right after finishing the TV series. It is very tempting to do so (for comparison) but you are bound to be disappointed if you do so (due to various reasons to be discussed below).
Advice #2: Forget what happened in the TV series about Kano and Minagi as they play no part in the movie so do not hope for much of their appearance in the movie. (same for PIKO PIKO!)
Advice #3: Do NOT view the movie as inferior to the TV series. You are comparing French to English. The two are not even made by the same director!
Story:
It was a very good idea for Toei Animation to have both Kanna and Misuzu's story intertwined throughout the movie. This makes plot development much simpler to understand. For example, instead of the dreams that Misuzu often had in the TV series (as a way to link the past with present), she now uncovers the past through her field work. Two every different ways of presenting to the viewer "the story behind the story". There were also significantly more romance involved between Kanna-Ryūya pair and Misuzu-Yukito pair.
Art:
Since the movie was produced by Toei Animation, the animation quality will obviously be different from its TV counterpart. It is certainly not wise to compare the two (movie VS TV series) in terms of graphic (Kyoto Animation win by light years ahead). However, it is worth noting that the Movie does have some unique filming techniques that were quite impressive but those "still image" picture-book-like techniques were kind of overdone a little. In addition, for a 2005 production, the animation quality is really not that great (especially for a movie).
Sound:
Soundtrack wise, the same OP/ED songs were used for the movie as the TV series. So it should make most viewers comfortable watching the movie (not sure if it were wise to do that as it'll just make viewers want to compare the movie with the TV series even more).
The major difference from the TV series is that Yukito in the movie is NOT voice acted by Daisuke Ono, instead Yukito's CV in the movie is Hikaru Midorikawa (Heero Yuy from Gundam Wing, Kaede Rukawa in Slam Dunk, and Yusuke Yoshino in Clannad).
Character:
While it was enjoyable to see more character development and more developed relationships between the main leads, it was also sad that major characters in the TV series like Kano, Minagi, Michiru, Uraha, and even POTATO did not have much of a role in the movie.
It is worth mentioning that some characters' personality are altered slightly. What does that mean? For instance, the "love" between Kanna and Ryūya in the TV series was very pure (Ryūya's love was more like out of respect), but the movie version have a totally different take on that. Much the same between Yukito and Misuzu.
Final Impression:
I tried my best to not compare the TV series and the movie as much as possible (due to the simple fact that they were produced by different company), but I still find it hard to rate the movie as high as the TV production. However, the movie did give us some alternative paths that were not taken in the TV series (which was rather unique). It does not really matter which to watch first (TV or movie), but ultimately both have the same tragic ending (though the movie was less tragic due to Yukito being alive at the end ).
