The devs, I think, have stated elsewhere on this site that defense will be things like "defense versus AoE" and "Defense vs melee" etc, not "Defense versus burning" etc. Resistance, on the other hand will be "resist burning" and so forth.
This calls to mind the question of what the tanker resistances will look like as plotted on a bar graph. Each bar would be a different type of damage, the heights of the bars representing amount of resistance.
Obviously, everyone will want all of their resistances to be maxxed, or close to it via the use of Augments etc.
What would we, as game designers, want that "resistance spectrum" to look like when we compare different Stalwart sets to each other? I don't think you want all sets to look basically the same, because then that makes the choice of which set to take pretty pointless.
Some thoughts for making different sets behave differently, thus making the choice of which to take relevant:
1. Set has below average overall resistance numbers (across the board) when maxxed out, but is easier to max out (i.e. cheaper in terms of Augments needed) thus allowing you to allocate more Augments and slots to other things (like attacks).
2. Set is extra good against a few damage types, about average against a few, and below average against others.
3. Set is VERY strong against ONE type of damage and about average against all others.
4. Set is above average against most damage types, but exceptionally weak against one "kryptonite" damage type that you'll actively try to avoid. (I think most people would take this one and just avoid the kryptonite, which is why you maybe don't do this one, or else make the kryptonite a thing that either behaves randomly somehow or shows up even when you don't expect it to).
5. Set is pretty good across the board, but requires more resources to maintain, i.e. best Augments, a lot of slots, uses reserves a lot, leverages momentum, etc, leaving you very little to devote to attacks.
6. Set has above average resistances across the board but is unreliable in some way (powers have randomized durations, or toggles can have a chance to drop when you get hit by stun effects or knockback effects and need to be reset, etc).
7. Set has above average resistances across the board, but applies some debuff to the character (Stone Armor did this in CoX, as Granite had -move and I believe -Damage for a time).
R.S.O. of Phoenix Rising