I am out of residency, in private practice for 2 years, so there may be new data. I recall people saying 25% of residents do a fellowship.
Plastics is probably the most competitive. Neurootology is really small (~13 spots?), so it can depend on how much interest there is.
Rhinology fellowship seems unnecessary if you just want to do sinus cases, because most residencies teach FESS well. But, could be good if you want to do academics and be involved in rhinology research or endoscopic skull base surgery. It would also help market yourself in an urban area, or if you just like doing sinus cases and not general ENT. It seems like many rhinology trained folks do some general, also, though.
Peds fellowship is kind of similar to Rhinology. Good for academics if you want to be the end of the line for rare and challenging cases, or research . Also a marketing tool /ploy, ie if you want to attract insured children in urban areas.
It seems like field - specific research is generally needed to show commitment for a fellowship, and faculty willing to make a phone call for you.
Monetary factors are not really worth considering as a MSII. The field could change in 7 yrs. It is complicated with regards to if a fellowship is a good financial decision or not. General ENT has a lot of potential for success. I started solo out of residency, merged with someone after a year. I am doing better than most salary offers. On top of that, I have complete control over my practice. I would hate being judged by a hospital system and reporting to the head of otolaryngology in the hospital system.
There seems to be a lot of people saying the shift to employment is going to continue. I would argue that in many if not most markets, private practice remains a much better option, financially.