I'm just gonna purchase taps and dies for my most commonly used metric screws. I want them to last for as long as they can (not that they will be used so often, I just want them like that). I also will be hand cranking them, so actual high speed cutting won't be done. So which is better for my application, HSS-E or HSS coated in TiN??
HSS-E steels are HSS steels alloyed with Cobalt.
Cobalt makes steels harder and increase the heat resistance. Detriments are the increased breaking susceptibility and reduced toughness. Source in German
Since heat resistance is of no concern in your application I would choose TiN coated HSS taps.