I'd suggest using the best quality lab that is convenient to you...preferably a local pro lab where you can actually talk to someone face to face.
The products you sell, are a representation of your work, your business and yourself. Do you really think it's worth saving a couple dollars by having your prints done at a cheap consumer lab?
The actual cost of a print should be a very small fraction of the price you charge for it. So get the best quality prints you can.
My intention is not to save a couple of dollars. The reason I asked the question is because I took a beginner's photography class last year and the guy who taught it is a professional photographer. I live in the Chicago area, and this guy said that most pro's in our area (including him) use Sam's Club or Costco. I was a little surprised by that, so that is why I started this thread.