[spooneybarger]

If you want to add to your skill set, I would think you have a couple possible motivations...

motivation a- expand the mind.

If that is the case and you know c/c++, I would say, go with scala as it is the farthest afield from what you already know.

motivation b- develop a marketable skill.

If that is the case, then I've always been of the opinion that it is better to develop skills that others don't have. If you learn PHP, you've learned a commodity skill. If you learn scala, you are in a select few for the time being and although their might be fewer jobs, there will also be less competition for them and you can probably charge more. I don't know what the going rate for Rails developers is now but a couple years ago, it was crazy high because the demand outstripped the supply.