it doesnt matter actually since dpi affects the output dimensions. the number of pixels doesnt change with dpi. you can set 72dpi or 300dpi in photoshop later. which you choose depends on what device you are using the jpeg on - 72dpi for display, and 180, 240, 300dpi etc for printer.
you cant...and as what we said above. it doesnt matter because dpi is only used for output resizing. you can happily bring your original 72dpi jpegs from your s3pro to those photo shops and they will churn out nice prints for you.