They fit exactly the same printers — the XL simply holds more ink. If you print often, 26XL works out cheaper per page and means fewer cartridge changes. For light use the standard 26 may be enough. You can run one of each in the same printer without any problem.

None beyond capacity. Polar Bear is simply the picture Epson prints on the 26-range packaging so people can recognise it in a shop. The XL holds more ink; the fit is identical.

No. The 26XL range uses separate cartridges for each colour, so you only replace the one that has actually run out. A multipack normally works out cheaper than buying four singles, so it's worth a full set if you print regularly in colour.

Yes. Refilling is the cheapest way to keep a XP-600 or XP-810 running, and it keeps empty cartridges out of the bin. There is an extra accessory you will need called a chip resetter, this is used to reset the ink level on your ink cartridge. These are around £25 online.