After a river cruise, trying to decide whether to fly home from Nuremberg, which for me will require an overnight stop, or travel to FRA or MUC to fly home with only one stop required. How easy is to get from Nuremberg to either FRA or MUC by train?

Train travel in Germany is very easy. There will be a connection that will take you right to either airport. Munich is closer. What is the timing? You will likely have to spend the night if you have a flight leaving in the morning. Will add times and links in a bit.
ETA: looks like about 2 hours to Munich airport. about 30 euros. And just under 3 hours to Frankfurt airport for about the same price. One change to Munich (central station) no changes to Frankfurt. Trains leave pretty often.
https://www.bahn.com/en is the website, easy to navigate.

I am based in the Nuremberg region and I use NUE as much as possible. The reason is that it is one of the easiest airports to navigate, has a great airport hotel (Moevenpick), efficient check in, a really nice lounge, and the security often takes only a few minutes. The feeder flights to the various hubs are fairly frequent and well synced with the transatlantic flights (at least if you're on Skyteam or fly Lufthansa). This makes the airport experience much more pleasant than MUC or FRA. While you can reach both of them in about 3 hours using the ICE (plus subway), if you are already in NUE, it's really not worth the hassle of the train ride, the much higher cost of the airport hotels, and the inconvenience of the much larger airports.

Thank you both. That info helps.

It's very easy to plan your connections (and buy tickets) if you just go to www.bahn.de, and switch to English in the upper right corner where it says "Deutsch" As others have said, Frankfurt Airport has a long-distance train station, so in that regard is easier to get to, while in Munich you will have to transit to the suburban train (S-Bahn), which is underground from the platforms where your long distance train from Nurember will arrive. Frankfurt is definitely more straightforward. High-speed trains ("ICE") to both cities run pretty much every hour. Nuremberg is halfway between Frankfurt and Munich, so every train connecting the two cities stops there

While it is easy to get to FRA or MUC from Nuremberg by train, it is very wrong to say that every train connecting both cities goes there. The routing of roughly 50% of the trains is via Mannheim-Stuttgart-Ulm-Augsburg.

You are right, it completely slipped my mind that there's the "other" route too. But in any case, as related to the OPs question, I think there's quite enough trains to either of the cities, which is what I was hoping to illustrate

Just as a recommendation get a "bahncard probe 2 Klasse". It will cost only a fraction and enables you 25% reduction. Usually already is profitable after 1 ticket.

While it is too late, since it was last year, would that have worked for a Cologne-Paris trip? I just assumed that a one time, one way trip it would not have been cost effective. I will file that away for future reference.