I have this same problem -- there are a lot of interesting languages out there that I'm interested in trying, but I always keep going back to languages I already know because:
- I have work to do; and
- it's hard to objectively compare language merits in the short term or for trivial projects.
I was thinking that the solution to this is to have one program that I understand very well implemented as well and completely as possible in a language that I feel proficient in, and have that be my reference. Then, over the course of a couple of weeks (a month?), re-implement the same program in the new language and strive for the implementation to be as idiomatic of the new language as possible. After all, if you're still thinking in the old language but just using the new one's syntax, what's the point?
I feel like this would give you a lot of data to make a reasoned decision -- you can compare language features and how the implementation works in one versus the other; time to implementation (LOC, maybe?); how much of a mental shift the new language requires; the toolchain around the new language; etc.
The problem is figuring out what the reference app is, and having the stomach for implementing it over and over again. Tetris, maybe?
But, back to the resolution (and partially touched on) -- I don't think a week is enough time. A month is even cutting it close, IMO.