Which one is a better pet, and why?
It depends on your lifestyle. I don't know about cats but they don't seem to be very friendly or all that playful or exciting. Dogs on the other hand have so many things they can do and usually don't take off running when you approach but instead eagerly await your presense with a wagging tail and slobbery tongue. But they are a lot of work.
I have one of each and wouldn't want to choose. It's like asking what tastes better, apples or oranges? I like them both. My kitten greets me at the door, plays long and hard with us and loves to be loved. My dog greets me at the door, provides me with much needed exercise and loves to be loved. Why choose?
I have had both in the past and have to agree w/ Quirky that it depends on if you're a dog person or a cat person & your lifestyle.
IMHO, cats are much easier to train. But, I think the time it takes to train a dog is worth it & I love to see my dog do something & know that I taught him how to do it. Cats are more solitary animals in most cases and if not solitary, calmer & less interactive, from my experience, and dogs are more interactive for the most part & love being w/ ppl. For example, when I had both a cat & a dog, I would be at the computer & the cat would curl up in my lap & go to sleep while the dog would bring me a toy & want me to play w/ him. So, if you are the type of person who doesn't have the time or patience for stopping to play while working, etc., a cat would probably be the best pet for you.
I loved both of my cats & miss them very much, but right now my dogs are so much fun & keep me on my toes! Also, I think dogs are great company... My DH works construction out of state & is sometimes gone for 3 weeks @ a time & I love having someone here (my dogs) to come home to when DH is gone. I do hope someday to get another cat, but right now just isn't the best time since we are planning a move soon.
Dogs are more responsive but tie you down way more and are way more work and expense.
Most cats have very different personalities (some are lovey, some are not. If you get one young and give it A LOT of attention as it is growing, you will usually end up with a nice pet.
For indoor cats, just feed them scoop their box and love them. Dogs involve, feeding, walking, scooping poop outside, etc...
For myself, I know I could never give a dog the attention it would want since I have two young kids. My cat is content to wait until the kids are in bed and curl up on me while I watch tv.
I prefer cats -- they just seem a lot easier to take care of. Dump some food in the bowl, and scoop the litter every day, and that's it. No walkies, no special training. I like the ability to just be able to go away for the evening or the weekend without having to worry about what to do with the dog. The cats basically take care of themselves.
Downsides -- they do tend to barf up hairballs. We got woken up last night at 4am to the sound of one our cats throwing up in the hallway. He trailed it all the way down the hall. Lucky us.
As far as affection, like stir_fryi said, it depends on the cat. Our male cat adores me, to the point of obsession. He comes up and sits next to me, leans against my arm and gazes adoringly into my face while creeping slowly into my lap. I pet him for a while and then push him away. And then he creeps forward again. It gets really, really obnoxious sometimes...but I do love him :) Our other cat was semi-feral as a kitten, but she's really mellowed as she's gotten older, and has turned into a lap-cat as well.
Dogs are better for me, because I am allergic to cats. ;-)