Real HP ink is a bit more expensive, but lasts for ages. Top quality too.
Previously, I learnt the hard way when buying inferior, cheap ink. I thought I'd be getting an 800 page, 'overfilled' cartridge compatible with HP 339. Unfortunately, the quality was not as good as original HP 339 and the cartridges only last a few hundred pages. Very poor!
I would recommended buying these original HP 339 cartridges, which are brilliant quality and last for ages.