Since concrete work will be around for a long time, it's especially important to be careful of choosing any contractor based on price alone. A crappy job that can be unsafe or damages easily costs more to fix than if you paid a decent contractor the first time. A good contractor won't cut corners or use low-quality materials or rush the job so much that it can't be done right. You want someone who takes pride in their work and is as concerned about the customer as they are about the concrete. Check with past customers to see how their experience was.