I think you are looking at this from a wrong angle.
Language feature debates are esoterical
The presence of a language feature or the absence of another is not what makes a language useful. And even when your take a single feature, people might actually disagree on whether it is in fact an advantage or not. Personally - without wanting to argue, let alone claim I am right - I think the lack of static typing is a design flaw. I also think the static type system Java has serious design flaws. Same applies to operator overloading:
I saw `cout' being shifted "Hello world" times to the left and stopped
— Steve Gonedes
There are countless religious wars fought about whether a language feature is good or bad. And in the end, there's probably no such thing as a good or bad feature. A feature can be well executed and compose well with all other language features. That's what makes a language elegant. Please note that elegant still doesn't mean useful, which is why I think you're looking at this from the wrong end.
Talk is cheap - code is code
To convince your counterpart, you will actually need to implement working solutions for a sufficiently realistic problem from the domain at hand. So will they. And than you will have to choose sensible metrics to compare both solutions. As for "Python vs Ruby", my guess as an outsider (I don't particularly care for either) is really, that you won't find much of a difference. There's claims that Django is faster than Rails. If you can actually prove that and if that speed difference is important, you might have a solid argument at hand. Other than that, I don't think any of you will be able to establish superiority of one language over the other. It'll just come down to a matter of taste.
Now as for comparing Python to Java, I think you should be able to plausibly demonstrate higher productivity with Python. But that's really for you to accomplish.
Either way, when comparing toolchains (which is what this is about in the end), any side of the argument must provide hard evidence on how their toolchain makes real life problems easier. Use that evidence to either prove that one toolchain is superior or toss a coin if all else fails. You either win by getting your way or win by getting a chance to learn something new.