Although "Vampire Lovers" is closer to the letter of LeFanu, I think "Blood and Roses" captures the brooding, melancholic, delicate tone of the novella better. Also, I think this is by far a better made, better acted and overall more entertaining and emotionally powerful film than the Hammer version, and it´s also my favorite horror film.
I usually don´t like to compare books to films, as I feel some people get a bit too defensive of the literary sources and simply hate the film without evaluating as a film itself (they are different mediums after all), but regarding the changes made from the original story, I think making Carmilla a heterosexual rather than a lesbian was an interesting choice, since we never get to know for sure why does she seduce and then kill her victims. Georgia on the other hand, is the repressed lesbian (in the extended French version, there is a short scene of Georgia in her room after the greenhouse sequence, in which she is clearly moved and confused by what happened), who does not realize that the girl she grown to love and trust, is in fact trying to kill her. The focus is changed from Laura to Carmilla, and thus making this change plausible and not all that different from LeFanu.
The theater is like a faithful wife. The film is the great adventure, the costly, exacting mistress
That's a good point about defensiveness. I know someone who hated Larry Fessenden's "Wendigo" just because it had the same title as but was not drawn from Algernon Blackwood's short story, to give an extreme example. What works in print doesn't necessarily work well in film, and vice versa. It's perfectly reasonable for a screenwriter and director to use a source story as a jumping-off point from which they create something new. These works aren't sacred things that demand worship, no matter how great they may be.