Ah, one of my favorite discussion topics...
First off, I fully admit that I am a huge fan of the 2005 movie. I think Joe Wright is a fantastic director (go watch the Soloist and Hanna for further proof), and he did a wonderful job on the movie. He had to know that no matter what he did with the movie he'd be criticized by Austenites because he had to cut scenes from the book, yet he took on the project and did a great job.
First, it needs to be said that both productions are really quite good. And I have no problem with people who prefer the BBC production. Colin Firth is a great actor and he's proven that over and over again. But I felt that Matthew MacFayden really sold the tortured soul of Darcy so well, I just love and prefer his performance. I also prefer Keira's spunkyness to Jennifer's, when I pictured Lizzie in my head, it's Keira i think of.
I also liked 2005's Mr and Mrs. Bennett. Joe Wright cast those roles beautifully. Sutherland really caught the beaten down Mr. Bennett, and Blethen's desperation was beautifully portrayed.
Surprising to me, I did not like the 2005 portrayals of Lydia and Kitty. Surprising because I think both Jena Malone and Carey Mulligan are terrific actresses, but both of them turned in cartoon-like perforances IMO.
And I think Tom Hollander just killed it with his portrayal of Mr. Collins. Other than the fact that in the book Mr. Collins is tall, and Tom is clearly not tall, I think he did a terrific job, bring obsequence to new levels!
One final thing, there is a clear difference in the production of a big time Hollywood movie and a BBC miniseries. The cinematography in the 2005 film is breathtaking. Plus the sequences filmed in the Ball scenes was just brilliant. But the BBC film does take good advantage of the much longer film length, so if you like the depth of the story told, well, the BBC film is a good choice.
Suffice to say, I can't blame anyone for preferring one version over the other. But for me, I can watch and rewatch the 2005 film over and over again.