The choice does matter. A lot. When I started with ruby, I had to decide between 1.8, or 1.9, or jruby, or several other choices. Then I had to pick sinatra or rails or merb or whatever. mongrel or passenger or thin or unicorn. It didn't really seem like the choice would matter, but then you run into some tiny annoyance, do a bit of research and find out that some other framework doesn't have that problem. So you switch, only to find a new problem. This goes on for a little while until you realize that you could write the whole damn thing from the ground up in lua.
The time spent just researching whether it's a bug in my code or their code starts to eclipse the total project time if done in another language.
If it's early in the life of a project and you don't have much experience with the framework, tiny annoyances are a warning sign. If the easy things don't work the way I like, what happens when I try to do something hard? The cost of switching is low.
I used merb for all of about 10 minutes before giving up on it. I fooled around, created a couple very basic crud pages, and it worked for a bit, then I got nothing but error pages. root cause: I had created a new class called 'fish'. That opened my eyes to the fact that were was so much automagic fairy dust sprinkled about, if anything serious ever went wrong I would never figure it out.
You have to choose between all the same kind of stuff with PHP too. The position was "I'm using PHP, and I can't switch to another language because I am afraid I'll pick the wrong one". That position is silly, because anything you pick is guaranteed to be better than PHP. There's no "wrong" between python and ruby, they are for all intents and purposes the same.
As for your specific case, writing your own framework in lua doesn't make ruby a wrong choice. You could have written your own framework in ruby too. Or perl, or python, or even one of those dreaded compiled languages.
The time spent just researching whether it's a bug in my code or their code starts to eclipse the total project time if done in another language.