If you're sitting around asking yourself, "Are we ready for kids?" and conclude, "Let's adopt a pet to find out!" you're not ready for either.
I take issue with the idea that pets are good practice for children. The similarities between parenting and animal companionship are thus: you have a being is dependent on you for sustenance, nurture, and love. But the form of meeting those needs is significantly different between the two.
I imagine nothing could really prepare you for what it's like to parent 24/7, but pet ownership isn't it. I dote on my cats, and I have a litter of kittens I'm raising from birth, but caring for them is not like caring for children. I can leave my cats alone while I go to work. So can my friends who own dogs. When I correct my cats, its for shredding the carpet. And that correction is the form of spraying water at them or making a loud noise that startles them, not having a heart to heart about obedience and responsibility (how I wish I could!). My cats will die in about 15 years and I can choose to start the cycle all over again. A baby cannot be left unsupervised for hours at a time.
Adopt an animal when you've done your research, talked to other pet owners, and spent time with the kind of animal you want. You'll have a good understanding of whether you're ready for that commitment.
Have a child when you've done your research, talked to other parents, and spent time with children. You'll gain a better understanding of whether you're ready for parenting.
Don't adopt an animal to test the waters of your responsibility.