There are a lot of different CMYK spectrums out there. When designing in Photoshop or GIMP, you may see a bunch of different options
It doesn’t necessarily matter which CMYK spectrum you use because this information is not saved upon uploading. Your uploaded image is copied onto the different product templates and an RGB file is sent off to 3rd-party printers who will then print your product.
There won’t be any major color differences but each printer is different and there are several factors that can affect printing and colors, such as the ink the printers use, the temperature of the room, how products are hand-fed into a printer. The printer itself will match the best possible CMYK output color to your file’s RGB color.
So to simplify:
Preview the work in CMYK (so that you have a general idea of what colors will look like on a product) and save the work in RGB (so that there is a minimal shift in colors during the transfer and printing process)