It will depend on what kind of sights you prefer. If you like the sights that they have on, and don't feel like dealing with shipping your slide back to Glock (wait time?) , then I would say that buying them installed is a better option. Just as long they aren't jacking the price up too much due to having nights sights already installed (I have seen it before).
I don't really care for the factory night sights, but they do work. I use the factory night sights on my duty Glock because I have to(Dept. Policy). I use Warren Tacticals on my personal G17 and G21. To me, there are better sight options out there, but use what works for you.