Side-by-side comparisons aren't really all that appropriate. Get yourself a copy of the Sailmakers' Apprentice and read it--you'll then understand quite a bit about sail construction and materials as well as repair. You might be happy at that point when interviewing lofts because you'll be comfortable with your own knowledge of sails and sailmaking.
Local sail lofts end up with loyal followers. You can ask cruisers or yacht club members near you and you are likely to find an affinity for a particular local sail loft. To a certain extent, sails for a production boat are basically a "commodity" type purchase. Your boat owners' group is a good place to start asking questions of others who have used your boat as you plan to use it.
Beyond that, local to where you are start asking questions. I also have found that the good sailmakers aren't in any hurries to jump through hoops and get quotes to you for your new sails. They're in high demand and busy. If you wish to go with a "commodity" sail, you can contact Rolly Tasker, btw.
Here's a couple of our previous CL topics on sails: