My Opinions:
1) I love D&D Beyond for character sheets, and dice rolling, but they lack in the virtual tabletop arena.
2) Roll20 is a good solution for getting a visual reference during a campaign, but it has a bit of a learning curve.
You can purchase digital copies of D&D books through either D&D Beyond or Roll20. Personally I like giving my money directly to Wizards of the Coast LLC (the creators of 5e) I feel it's important to mention both site have free options or subscription plans for people who want to GM or want more options than the basic players.
D&D Beyond, and Roll20 duplicate many features of and I find paying for two services that provide overlapping services feels bad. So using something like Owlbear Rodeo is perfect for my group. your mileage may vary
