Huge. That's the difference between printers and their equipment.
Even the grade of photo paper they use is important.
Do a very simple and cheap test. Take the same set of files and send
them off two a couple of local 1 hour places (Walmart, Eckert, etc) and
send to the mail-order places. Take a look and decide who you want to
use. You can do the whole thing for $25 or so. You will be amazed by
the differences. When you send the files, make sure you send images
with a variety of subjects, lights, etc.
Also make sure you send to places using different machines (Kodak,
Fuji, etc.).
I think the differences in the highlights, lowlights, and skin tones
will amaze you when you see them side by side.
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