I'd like to take some time to learn more about dynamic languages built on top of the DLR and I'm not sure which language would be better to learn.
Having limited time, I really only have time to look learn one of them.
Any opinions on which of the two (Iron Ruby or Iron Python) would be more useful in the long run?
I just want to mention that there is also a DLR version of Javascript(JScript), which is my personal fav.   If you are looking for a new language to learn for dlr use, I'd suggest ironpython as mentioned, it is farther along in terms of the dlr.  Python is also fairly popular outside the dlr for gui programming, and offers Django for mvc web apps.
This is purely subjective, but I think that ruby popularity is waning a bit.  In the long run I feel it will be like perl, used and respected, but a  drop in the bucket compared to other options.  I happen to really like ruby (and perl), but wouldn't suggest it as a new path for your intended purpose.
Which Dynamic .NET language makes more sense to learn, Iron Ruby or Iron Python?