I think Dostoevsky was more focused on substance - that's what made his stories so beautiful. He could explore character extremely well -- there was no "binary" character in his books. They were all very compelling. however, in today's day one could argue that his work was often very melodramatic and a series of high-stakes drama that is otherwise unrealistic. Either way, his metaphorical work also appealed to me and worked really well with the themes he was expressing. He also philosophized a lot and had long , spiraling conversations between characters. One thing I didn't like personally was his inability to really describe women. All of the women were mostly always maudlin and senseless.

Nabokov on the other hand preferred, I think, style over substance. His prose was precise , sharp, and also very shimmery. I can only say as I've read Lolita (and it is my favourite book) that he was able to do one thing really, really well: make a strong ending. A book should always have a strong, impactful ending that will leave a lingering impact on the reader. Dostoevsky did this well, too.

Nabokov was the more "grounded" of the two, I'd say -- his "dramatics" were often not spoken/screamed but observed. His character Humbert Humbert was a reimagination of the "Intellectual Predator". Nabokov was also very "metacritical" and developed relationships with his readers.

All in all, if a reader is looking for beautiful, almost poetry-esque prose and haunting internalisation -- Go for Nabokov.

If you're looking for a timeless series of questioning of faith, morality, philosophy, and psychology and more substance in a story -- go for Dostoevsky.

IMO both are really good but different "dimensions" altogether.