I am toying with INV or ELA and leaning more towards INV. I posted a build a while back softcapped to S/L/E/N with 1 foe in Invinc.
I think it is potentially less clicky than ELA as well. I see the benefits of ELA esp on the endurance front, but I keep thinking INV may actually be tougher end-game (assuming Aid Self / Rebirth).
My sense is with Cardiac the endurance issues won't be as bad, and can support INV. But I think we need MIDS updated to really figure it all out. I'm still open to being convinced on ELA.
Damage auras shouldn't be ignored either.
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This may seem like a minor thing, but I personally like Invul a bit better due to the +tohit you get from invincibility. TW hurts more from a missed attack than any other set (not only you've just wasted a long *** animation and a chunk of blue, now you have ANOTHER attack that you must put through the long *** animation before you enter momentum phase. Not to mention the hit you take to your overall DPS if you miss an attack WHILE in momentum), so every bit of +hit helps.
I'm generally wary of taking heavy-duty resistance sets on scrappers - Invulnerability scrappers, for example, can go sort-of-but-not-really Unstoppable, while brutes can go full-on tanker-level Unstoppable.
Willpower is scrappier, certainly, though if you're a fan of Strength of Will, you'll again hit your head on the resistance cap. Also, regeneration likes health.
This doesn't apply (or applies to a much lesser degree, as brutes are able to push 80% smashing/lethal resistance with Invulnerability) if you're not a fan of the crashing T9s, of course.
Putting the secondaries aside, TW feels like a rather brute-friendly set to me - simply because it rewards you for hanging out with lots of enemies (by letting you swing at all of them, and by building momentum if you hit any of them!)
Incidentally, this also meshes beautifully with both the Willpower and Invulnerability auras... meaning I'd go brute to get triple rewards from the "hang around as many enemies as possible" style of fighting.
Having said all that, the possibility of crits on the many AOEs should not be ignored -- or rather, if you ignore crits in favor of higher-health / resistance on Brutes, you are making a decision in favor of survivability vs. wet-your-pants-zomg-arc-of-destruction-just-wiped-like-8-dudes crits 
I'm liking my TW/WP scrapper so far, and I'm having better survivability in thE 20's unslotted than my friend playing a brute with the same powersets. I've found a combo string that can take out an orange lt rank enemy in one chain, if everything lands no criticals, at level 21.
I have a TW/DA, and I have no problems with end at 28. I only have two more toggles (tough/weave) to add, but I dont have a numina yet, pp, or my second shifter. It seems to be a lot more fun with the pbaoe damage and spots to slot more +ranged and +melee def sets.
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For scrappers, WP is superior to Inv in my opinion.
The main reason is the aggro, as Inv gives insane amounts of aggro when compared with WP, which gives you "just enough" for mobs to stay put. Also, Inv is more suited to Tankers and Brutes in a sense that they can take better advantage of Inv's strengths, with their higher HP and resistance caps.
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I'm playing TW/WP and it seems pretty tough. Only level 18 atm. I really love the pounding I can lay down after hitting BM. The set does a good job of making you feel like a bad ***.
K, my main is a SS/Invul Brute. I really love Invulnerabilty's insane toughness. But I hear TW eats the old end bar. I enjoy a good Will character as well.
Will a TW/Invul be brought low gasping for breath, or is that a good path to go?
Also, do we have a definitive answer on whether TW is better on a Brute or Scrapper? And why?