I have both. 2 cats and a dog.
The cats came first, both so affectionate, sit on my knee purring, they chat to me, they are a calming presence. They are independent, funny, and best of all they aren't needy. They can be left for a whole day, in fact when we go on hols we leave them and our neighbours feed them for us. They don't cost a lot, they eat very little and vets bills have never been huge.
Fast forward 10 years and I decided to add a dog. Dog is so needy, can't leave her for more than 4 hours at a time, costs me so much money to feed, insure, vaccinate etc. Makes me go out walks when it's raining, cold, I'm sick, I can't be arsed. Tortures me when I'm trying to work or relax on the sofa. Vomits on my rug at least fortnightly, means we tip toe around at night cos god forbid we wake her, constantly begs at the table or if we are eating ANYTHING.
But we absolutely love the bones of her and would never be without her.
Personally, start with a cat and see how you go, dogs are a huge commitment, I mean really huge, I was not prepared after a lifetime of just having cats