Iro accounts for 10 extra health per level when you're surviving on med-packs, or 20 per level if you get a nice stock of boosted life going. TaN, on the other hand, absorbs 1 damage per attack, to a minimum of one damage. Since you had P-onyx, I'd say you made the right choice. (Although, the fact that TaN also absorbs floor damage can be hella-useful.)
I take it you weren't attempting the Shottyman Diamond: still, looks like you did rather well without any mods or special shotties. It's not a bad idea to use the combat to sweep through the first six or so floors, since leveling will get harder over time, and you aren't relying just yet on Rel, therefore you can level HR first (DM is optional).
I've had a few good AoSg runs so far, using each master trait, so I'm just going to attempt all-or-nothing from now on and play MAD. It's the only guaranteed method of getting enough damage later on and, as you can't rely on Mortuary for a min-kill raise, you'll have to be consistent throughout the entire game. It blocks off Fin and HR, both of which are almost too good to pass up in N!, but Iro/SoB can keep you going well enough. (Besides, in MAD your primary weapon is the combat shotgun, which means lots of killing without leaving cover.) If, on the other hand, you get an early super or plasma shotgun, you're better off heading towards MFa instead (thank goodness TaN doesn't block it).