Objective reasons for using Python or Ruby for a new REST Web API
I think they are fairly evenly matched in features.  I prefer Python, but I have been using it for over a decade so I freely admit that what follows is totally biased.
IMHO Python is more mature - there are more libraries for it (although Ruby may be catching up), and the included libraries I think are better designed.  The language evolution process is more mature too, with each proposed feature discussed in public via the PEPs before the decision is made to include them in a release.  I get the impression that development of the Ruby language is much more ad-hoc.
Python is widely used in a lot of areas apart from web development - scientific computing, CGI rendering pipelines, distributed computing, Linux GUI tools etc.  Ruby got very little attention before Rails came along, so I get the impression that most Ruby work is focused on web development.  That may not be a problem if that is all you want to do with the language, but it does mean that Python has a more diverse user base and a more diverse set of libraries.
Python is faster too.
So this thread is definitely NOT a thread for why Python is better than Ruby or the inverse. Instead, this thread is for objective criticism on why you would pick one over the other to write a RESTful web API that's going to be used by many different clients, (mobile, web browsers, tablets etc).
Again, don't compare Ruby on Rails vs Django. This isn't a web app that's dependent on high level frameworks such as RoR or Django. I'd just like to hear why someone might choose one over the other to write a RESTful web API that they had to start tomorrow, completely from scratch and reasons they might go from one to another.
For me, syntax and language features are completely superfluous. The both offer an abundant amount of features and certainly both can achieve the same exact end goals. I think if someone flips a coin, it's a good enough reason to use one over the other. I'd just love to see what some of you web service experts who are very passionate about their work respond to why they would use one over the other in a very objective format.