I'm a datapack creator; datapacks have much larger potential to add fully new content than add-ons. They're also good for small tweaks, of course, but huge content datapacks that add hundreds of items are not unheard of.

You can't really compare them because their for different versions of the game,
but I prefer datapacks

"on" is'nt capitalized in "add-on"

IMPORTANT NOTE: None of the code or technical syntax is tested/legit. They're just for illustrative purposes.

As you said, add-ons are BE, while data packs are JE.
As far as I know, custom models and textures are in external resource/texture packs for both of them, so let's leave them out of the question and focus on purely mechanics:

Add-ons are purely definition style (like HTML/CSS/CSV/JSON/XML). All data is defined in JSON and a new entity for example will only be a composition of traits. It's like the vanilla behaviour is exploded into a bunch of pieces and you can combine any of those elements into new compositions. There is no logic involved like you would expect with Mods, just never-before-seen combining of existing elements.

Datapack behaviour is defined through functions, which are files listing commands. It's comparable to functional programming like JS (okay, not really, what I'm trying to say is that it's not JUST a definition like JSON, HTML or XML). It's actual code with some (limited) logic. Of course there are still definition files, for recipes, advancements, loot tables, etc. But the actual behaviour is mostly implemented using commands. This makes it a little more difficult to make an entity "behave", but you have way more flexibility. Functions are not event-driven, besides the minecraft:tick tag that causes functions to run 20 times a second. The actual logic and behaviour comes from carefully filtering out entities in commands. Also functions can call other functions.

As a comparison, making a pig explode in an add-on would likely just require you to add something like "entity.explode" into a JSON array or object and that's it. Whereas with functions it looks like a command:
execute as @e[​type=pig] at @s if entity @a[​distance=..4] run function bertiecrafter:pig_explode/explode
And with explode.mcfunction containing:
summon tnt ~ ~ ~ {Fuse:0s}
kill @s

As you can see, the "execute" command is fired 20 times a second for each pig, only if there is an entity that matches the selector "@a[​distance=..4]" (which translates to "all players within 4 blocks").

Bonus, mod vs data pack: A mod would use a simple for loop (java code) instead of running a command 20 times per second as long as an arbitrary timer score is not 0. A mod would also be able to store entities in variables, making it super efficient to target the entity for a second time later on. A datapack would have to keep checking 20 times a second if all conditions pass and then take all entities and apply the same filter again to get to the same entity. Also the mod API is bigger than what can be achieved using minecraft commands. This doesn't mean that data packs can't create awesome things (browse PMC), it's just that sometimes you'd have to get creative to find solutions.

So final comparison:

	Add-On (AKA "Bedrock Mods" on PMC)	Data pack	Mod
Platform	Bedrock Edition	Java Edition	Java Edition
Creation style	Definition using JSON modules:["entity.creeper.explode"]	Limited functional programming function folder/do_something	Full object oriented programming entity.jump(); player.selectSlot(1);
Activation	On-load, perhaps there are ways to define filters for behaviour	Mainly every tick (1/20th of a second), filtered by selectors.	Event-driven. Filtered by if/else blocks, earlier defined variables and scopes.
Pros	Easy to get into for non-programmers.	Officially supported. Quick updates. Once you know what you're doing, not all that limited.	Efficiënt if you code it well. Almost anything is possible.
Cons	Terrible documentation Very limited in capabilities	- Requires creative thinking and some experiementing to get the hang of it. - Can be difficult for non-programmers and programmers who never worked with animation loops or any other kind of timer-based activation.	Guaranteed to break every update. Requires basic Java programming knowledge which is a steep learning curve for non-programmers. Also requires separate IDE.

First at all, mods are not official. As you said they require third-party APIs like Forge or Fabric. I think there are also mod providers for bedrock edition, but they are a lot less powerful, because C++ is a lot less compatible for such things than Java. Data packs and addons are official, and they are really a bit similar. Add-ons allow you to change things that data packs cannot like mobs AI, but they totally don't win. Because in data packs you can make functions tick, and with that you can literally tell the game what to do. Not just "explode when entity in range" but add whole mechanics. Also in Java there was the amazing 1.13 that made commands a LOT more powerful and better. Add-ons: I can change that pig! Data packs: I CAN CHANGE THE WORLD.