What's a good secondary for Spines?

First, to stipulate, I don't repeat powersets within ATs so Regen, WP, Fire & Shields are out.

Second, I'd like to stick to a concept, though you can stretch things pretty much where you need to.

I'm thinking of making a tech-oriented Spines/? scrapper using the new metallic Spines costume option. I'm thinking of a guy wearing a suit of liquid metal or something like that. So...

Spines/Invul - Fits the concept well. How's the synergy? I actually have relatively little experience w/Invul--my Invul tank is only in his 30s or so. I'm assuming Spines wants lots of mobs to attack, which is good for Invinc, so guessing this is a good combo?

Spines/SR - I've been meaning to reroll an SR scrapper (first one ever back in I0 was a Dark/SR, but it never worked out), but I don't see this as a good concept (yeah, the suit gives me enhanced speed... meh), and I don't know about how well Spines & SR suit each other.

Spines/Dark - Black liquid metal w/suitable costume options should make this work. Again, not sure about the synergy, though lots of mobs + Dark Regen always work together nicely. Plus the added damage aura sounds good. Wonder how OG would play out with Spines?

Spines/Elec - Currently have zero exp. w/ELA, but will be making an Elec/Elec tank for sure. Prolly similar to Dark in that it's resist based and has a damage aura, plus pretty much infinite end post Power Sink. Conceptually, electrified liquid metal Spines of Dooooooom.

Opinions?

I think with /ELA it depends on what you're fighting. With tough you get almost 60% S/L resist and well over the cap for Energy resist. Add in some good defense and recharge and you're gonna be tough. The heal isn't as good as /DA, but with enough recharge you can get it up near perma(you can probably get it perma, I just haven't tried yet).

Last night I took my level 22 Spines/Elec Scrapper on the Synapse TF and I was already at the resist cap. It was a ton of fun.

Good advice so far. So I'm leaning towards Spines/Dark atm. I can just about cap the S/L def on this build (with the lack of ST attacks, I'm having to 4-slot Brawl, which is just wrong on so many levels...), but end is looking dodgy, even w/the Nunima/Miracle uniques. Question: Is Cloak of Darkness not suppressed in combat? City of Data shows it as not suppressing at all, which would make my life a whole lot easier.

I'll have to put together a Spines/Invul build and compare, but yeah, I'm really liking the idea of 2 damage auras.