I think there are two main decision points:
1. How much are your comfortable spending
2. Do you intended to play with a VTT
I personally am a fan of DDB. I also like having the books. For me the books are for browsing, something nice to have on the bookshelf, and as a "backup" in case the digital assets go away. But I don't use books in play or even to prep, even moreso now that I'm not playing in-person games. Also, starting with Mordenkainen's, I actually started reading through the books using the DDB app instead of the physical books.
I like having the web page and apps because looking things up across multiple books is much easier. Having things hyperlinked with pop-ups of content is very handy--something you are not going to get with PDFs.
Also, I just started to have to wear reading glasses for smaller print, but I don't like having to wear them if I don't have to. I find it difficult to read the print books without glasses, but I can increase the font size, etc. in the app or on the website.
I always have my phone with me, so I can read through the books when waiting in lines, at the doctor's office, or on a plane and have read through much more material than I normally would with just the books.
When I was playing in person, I hated dragging a bunch of books to a friends house or the gaming store. Normally, I ran games from my house, so that wasn't a huge problem, but now that my job requires a lot of travel, it is nice to be able to carry all the books in my pocket, on my phone.
The character sheet is really well done and I prefer using it if I'm a player and when not using a VTTs character sheets.
BUT this brings us to the VTT question. If you have a preferred VTT and don't intend to change VTTs soon or often, AND if your preferred VTT has an official license (Roll20 or Fantasy grounds), I would recommend buying the content from the VTT provider, especially if you want the automations and other configurations done for you.
That said, DDB is the most supported in terms on connections to other tools. If you move from one VTT to another, there is a good chance that someone has created ways to integrate your DDB content with that VTT. But you are not going to easily take your official D&D content from Fantasy Ground and move it to Roll20 or vice versa.
For myself, I buy all the crunch books and some of the adventure/setting books in print form. I also buy the DDB versions along with some books that I only have DDB. I use Foundry, hosted by The Forge, for my VTT and use an add-in module to import DDB content into Foundry.