shutterfly has two sizes, 8.5x11, and 12x12. Apple calendars are 10x13.
I have received gift calendars from my two daughters. these are candid snapshots with their canon pocket cameras, a few iPhone shots now, as well as images from a couple of decent kids studios in the area that provide a cd of all the shots. They both look pretty good, but the apple one seems to be a little bit better. I like the size as well. I think both companies tweak the files to try and improve the quality, since their customer base is probably mostly like my two daughters.
a few years ago both HP and Kodak were pushing my company to install a digital press, HP's indigo and Kodaks NexPress. Was able to get a good look at the product as well as acquainted somewhat with the underlying technology.
I'm not sure where things stand today, but at that time Apple's provider was using NexPress which is a toner based solution and can handle more colors than just cmyk, HP's is more of a hybrid CMYK ink process. Both are good products and results vary based on the skill of the user, but I felt the Nexpress had a little crisper output and a little more snap. Shutterfly was using an Indigo solution.
I've also seen calendars from WHCC, more of a professional lab supplier that were quite nice. They were using NexPress back then as well, not sure where they are today. However, since they are used to working with professional photographers they may have a better color managed workflow option, including profiles to soft proofing.