Hullo, I like both cats and dogs but if I had to choose only one, it'd be dogs.
I will be generalizing in my post, I know not all dogs or cats are like this, but it has been my experience:
-Dogs can be bigger, which I actually prefer. I have anxiety about smushing smol animals.
-Dogs are always next to you ready to party like anytime. Lol, as long as you're excited, they're excited.
I like that dogs are usually at your heels or nearby and they're always very happy to see you.
-Cats have different behavior.
Not worse, just different.
They enjoy perching on high places, bask in sun.
Sleep a lot in the day.
Awake at early morning, late night.
That's cool and all but dogs will follow a human routine much easier sleep wise. Plus they are not usually climbing to extremely high vantage points.
-Many people who aren't cat people don't seem to realize that cats are easy to read once you get to know them. Ears going back, butt wiggle, tail flicks, etc. Cats will let you know very obviously when they dislike something. But most people are not aware of this.
Turning their back to you, showing their tummy (but not rub usually), slow blinking at you, kneading. It's all cute and if you know cats well they are just as loving as dogs but they're not usually the same silly vibe like OMGHIWOWEXCITED as dogs are and I like being able to continously pet a dog. Dog wants more pats!
Cats are often like.. 'Yeah, no. Half a pet was enough, thank you.'
-Dogs have owners and cats have butlers.
While you can walk and train cats quite well, it will never be as easy or routine as a dog.
I like going for walks accompanied by my furry companion, and training a dog, or even a pig, is way easier than training a cat (usually.)
With cats even if you find their favorite food ever it might not be enough of a motivation.
-Dogs don't randomly push items off tables in curiosity/sometimes just outright mischief.
Cats will look at you while doing said action, and flick their tail with satisfaction afterwards.
-While there are rare exceptions in the cat world, many dogs usually love to swim. Big plus.
-It's not important for me but I'll also add this: dogs do better with strangers, overall. Especially children.
Many people who aren't cat people had a bad experience when they were young with a cat that wasn't introduced properly to them. What I mean is, there was no warning or advice on how to approach the cat. They probably grabbed the cat's tail or got in its face, maybe they bothered it, and got scratched, slapped, or hissed at. It creates a lifelong aversion, which kinda sucks.

This rarely happens with dogs that interact with children that I've seen. (Well.. Except those tiny angry dogs.. Why are the tiny dogs the angriest?

)
Edit, forgot to also include, playtime with dogs is less painful in my experiences. They don't scratch nearly as much as cats do. Dogs are mouthy but it's extremely easy to teach them to be gentle in play. Might get the accidental scratch from a dog jumping or something but cats have drawn blood numerous times and they were just being playful. They just latch into you lol.
That was a really long post.. Sorry.
Cats have many upsides too, but you asked why I prefer dogs so that is for another time XD