I guess its up to you. I'd replace both speakers with the 50 watt models. Buy 16 ohms and wire them in parallel. You could just replace the blown speaker and as long as they are in phase, you may get a good sound, or maybe not so good of sound. The other advantage with replacing both, you have fresh speakers and you won't have to worry about the original speaker that may be getting weak.
Just my opinion.
pleat