Would recommend a dark fibre over a wavelength unless the price is very high. You can put your own wdm filter and run multiple wavelengths over it. And you aren

Would recommend a dark fibre over a wavelength unless the price is very high. You can put your own wdm filter and run multiple wavelengths over it. And you aren't relying on the providers wdm system to work, just make sure you get two fibers with physical separation if you need redundancy.