HP works great on Nightkin for me. AFAIK, Super Mutants do not have any DT, or at least a very low DT since they can't wear armor. They make up for it with a high END and STR rating and tons of hit points.
According to GECK, it's nightkin that have zero DT. Regular SMs (all variants) have 15 DT - even the named chumps like Marcus (who just have a lot of hit points).
15 DT is about medium-high for New Vegas.
The scale seems to be:
0 - no armor at all (maybe some damage resist)
1-4 - low-end (light) armor
5-9 - getting into the medium armor here
10-14 - heavier armor like leather
15-19 - the nastier critters, lighter weight guns just won't do it
20-24 - starting into the "you've gotta be kidding me" territory
25+ - break out the AMR and Mini-Nukes