Disclosure - I own a Langster, only ridden a MarkV a few times.

Both bikes steer and ride nicely. The Langster does not ride harshly at all. Its been a while since I have been on an IRO, and a lot of it could come down to fit between the bikes. So although my own bike (a sub 16.5 lb Langster) may feel nimble and smooth,.... its been a while since I have been on a stock Langster. I recall the bike is a lot less willing to go with the stock components. The wheelset/tires alone I dropped over a pound probably more. That makes a big difference in how things ride and how things spin up.

The Langster can be made into a rocketship. I did a group ride, running a 48/16 and was able to keep pace with no issue. I am absolutely sure that the IRO could do the same,... but it all depends on what you want to do wth the bike. Track geometry, to me, has zero value. I run brakes, I run a freewheel, I run QR's, and I will never run on the track. I do faster group rides with other roadies, I run SS on MTB as well as a FS MTB.

So ride both in stock forms. But if you want ultimate and outright speed on the street, I think the Langster is your better choice. It is a stiffer (bb) and lighter platform to build a bike out of. Ultimately, you can make the Langster a TINY bit faster than an IRO.... but if Lance were to ride either, it really would not matter.