Depending on how your GM rules it, Improved Invisibility and Armor at the same time probably won't help you too much, since the Armor spell itself glows (exactly what that means is not really that clear, but chances are it doesn't help too much with the whole invisibility part. A GM could go from the glow becoming invisible to the glow completely disabling the invisibility spell, or anything in between. That's something to discuss with your GM first at any rate.
Armor has the advantage of adding on to your existing protection, so it works well in pretty much all combat situations, and against a wide array of attacks. So that's a plus. On the other hand, you glow, and with "Geek the mage first" being all the rage these days, that probably isn't a good thing (you could always armor your tank and give him some fake foci, I guess). It's rather costly in Drain though. Altogether, I don't think this spell is much worth it, mainly because of the huge target it paints on you.
Improved Invisibility, on the other hand, does mostly the opposite. It's an extra layer of security, but doesn't add to your normal one - while getting 2 hits on Armor is kind of lame, but still useful, a 2-hit invisibility is practically useless as, once it's pierced, it has no effect (and it doesn't stack with non-magical concealment). It's also useless against many magical threats, as astrally perceiving is a total defense against it. Still, a 5- or so hit Invisibility is quite effective against almost all mundane targets - and you'll face those the most - with relatively low drain. And on top of that, unlike Armor it has a lot of out-of-combat utility (although Armor could be used to see in the dark, I guess?). So I'd go with II anytime.
Note: when I played a mage in SR4 I had both of them, and after a few runs I practically never cast Armor. The lack of subtlety just isn't worth it. In SR5 I didn't take the spell for that reason (it's mostly the same) and while there's been situations where I'd wanted an armor buff, I've yet to run in a situation where I missed the actual Armor spell itself.