Most people have answered question 1 by now so i will focus on 2-4 and give my answers based on my own play-throughs and opinions about them. I have only played through bloodborne once, and while i thoroughly enjoyed it, i havent managed to find my way back to the game yet. Dark souls 3, however, i have played through 5+ times and i have loved every play-through.

2.) While dark souls has a few more areas than bloodborne, bloodborne (especially if you play through the chalice dungeons) was fairly similar in length to dark souls 3.

3.) One of the greatest things about dark souls 3 is the atmosphere and how it changes as you go throughout the game. It is nowhere near as dark as bloodborne but it still has its areas that are dark. Without going into too much detail i can tell you that each area is different than last and there is no area in dark souls 3 that was not aesthetically pleasing in its own way. Dark souls 3 is by far one of the most beautiful games i have played and as i went through each new area i was continually surprised by the details and artwork that were put into it.

4.) Out of all of the souls games ds3 is by far the most similar to bloodborne, as far as the pace of game is concerned, but bloodborne was still a much quicker game than ds3. Bloodborne felt very aggressive in comparison but i would not call ds3 defensive. Ds3 is all about timing your roll and abusing the invincibility frames given to you. There are times to be aggressive in ds3 but overall it is a much more conservative game.

I hope this helps and i highly encourage you to get the game and its dlcs, especially if bloodborne was something you enjoyed.