It really depends on what you want to do with the tools. Are you going to only be a player or also a DM? Do you plan to play via VTTs (virtual Table Tops)? Do you want primarily a character builder or do you need other tools. What kind of tools?
If you need a VTT, then you need to go with Fantasy Grounds or Roll 20, at least to a point. I've not used either (although I do have a free Roll 20 account). I've heard that the character tools on both are limited/frustrating to use. Only Roll 20 appears to have a free option that would let you try out the tools.
Hero Lab only has the SRD for 5e, and you have to pay for that, where as the SRD is free on DDB (DnD Beyond). I don't know if you have to pay for SRD on Roll20 or Fantasy Grounds.
Many of Roll 20's versions of the official 5e books are more expensive than their DDB or Fantasy Grounds counterparts.
DnD Beyond (aka DDB) has the following features:
-character builder
-ability homebrew all character options except full classes (subclasses are homebrewable)
-compendiums: online versions of the books, with built-in hyperlinks.
-filterable listings of spells, monsters, and magic items. (Want to see all the 3rd level abjuration spells for wizards, all the undead with CR between 2 and 5?, a magic weapon that deals cold damage? The listings are a great tool)
-DDB is the only service that lets you buy things piecemeal instead of forcing you to buy entire books. For example, I have the entire PHB on DDB, but then I started buying piecemeal: the Tabaxi race, the Storm sorcerer, etc.
DDB does not provide a VTT, and it's campaign management tools are very limited currently.
There are folks who use both DDB and Roll 20 or Fantasy Grounds. A Roll 20 user has created an import tool to import characters from DDB into Roll 20. Maps from DDB can be imported at into Roll 20, maybe also into Fantasy Grounds.
Obviously, I'm a DDB fan. It does what I need and does it well. But, I don't need VTT tools. Both DDB and Roll20 have options that let you work with the tools for free. I'd encourage you to do that to see if they provide the features you want. If DDB is in the running for you, you might find
the buyer's guide I wrote helpful as you ponder what you need.