$600 for a Langster is a deal I wouldn’t pass up (I didn’t – that is why I own one).
It isn’t at all track geometry. The bottom bracket is too low, and the head tube is too high. Its fairly relaxed in that regard. What sold me on the bike is the way it handles. It really carves well, very light, and very responsive.
Similarly equipped the steel bike is going to be a few pounds heavier (both the steel langster and steel IRO).
If you are going to be beating on the bike (torn up roads), the steel bikes would be better, although the Langster rides very nice (mine came with carbon seat post and fork). The Langster really needs 23 or maybe 25mm tires, you can go bigger on the IRO – that might make a difference. I squeezed some 28mm on my Langster (because of the crappy roads I ride), but that forces me to be in the back half of the rear fork, and required some filing of the brakes to get the clearance needed.
I love the way the langster rides – I use it for short commutes with lots of turns and for longer centuries (the gearing is perfect). Ultimately it is how the bike rides that is going to answer your question.
Now, if I could just find a way to get the langster paint off of the bike – that look silly.