Upon arrival, I will have two days to spend before joining my river cruise group in Brussels. Would it be preferable to spend 2 nights in Bruges, or to attempt to split up time and do one night in Bruges and one night in Ghent? Have never been to either, and also would like to factor in being desiring of getting over jet lag (middle aged couple) Thank you!
Both great. Flip a waffle.
Stay in Ghent. Wander over to Bruges for a half day or so on the second day.
I would recommend staying in just one place and day tripping to the other. Now deciding whether to stay in Bruges or Ghent is the tough part!
The low-priced, frequent, train time between the two cities is not a factor. (A local bus ride, using the same stored-value transit card if desired, is necessary in both towns to get to the "old town".) Although it is nothing remotely like the movie "In Bruges", Bruges is very physically attractive. But I find Ghent a nicer place to stay, and it is nearly as attractive. Comparing them is very difficult, and controversial on all travel newsboards. Although it is not my favorite Beguinage, the one in Bruges is exceptionally large and famous. (My favorite is in Leuven.) Although there is important art in Bruges, it pales beside The Mystic Lamb altarpiece in Ghent, and there's a museum there too. With the KMSKA museum in Antwerp closed for renovation, other museums have gained temporary importance. Leave time for the many major museums in Brussels if that interests you. Do not rely on the river cruise to have time for museums in Brussels. It will be tiring to wrangle your luggage from BRU to Ghent or Bruges-the local trains don't have luggage racks at the doorways, but only over the seats. On the other hand, there is an hourly bus directly from BRU to the side of the central station in Antwerp. Have you considered going there instead? The train goes Brussels-Antwerp-Gent-Bruges (intermediate stops omitted.) I'm not trying to talk you out of Bruges, only to mention alternatives.
If you're under the age of 30, hands down, stay in Gent. Otherwise, slight edge to Gent, but mainly becauseif you don't want to wait in a long line to see Gent's #1 art piece, you have to be at St. Bav's cathedral fairly early in the morning.
Ghent! You'll want to see the Adoration of the Sacred Lamb Altarpiece, as Tom mentioned. Bruges is really touristy.
Both places are stunningly beautiful, but I prefer Bruges as there is simply more to do and see. I could literally spend a day in Bruges just sampling all the chocolate stores. Here's a taste: http://youtu.be/s-BYMPMhjpU http://en.wikivoyage.org/wiki/Bruges
Ghent hotels are cheaper overall. I think Ghent is quieter at night. Ghent is between Bruges and Brussels.
Ghent
This shouldn't deter anyone from the Mystic Lamb, but we were there in early August and the entire Annunciation portion is now gone to renovation. That is in addition to the Adam and Eve figures on the other side, the side with the Mystic Lamb scene. In their place are black and white replicas placed in the proper locations on the altarpiece. If you have time and inclination, I think the restorers are working in visible areas at the Kunst museum across town.
We stayed in Bruges and did a quick stop at Ghent on way to Antwerp; I'm not nearly as familiar with Ghent (or Bruges, for that matter) as many repeat visitors, but I loved the medieval Bruges in the early morning, before it got crowded; Ghent is more urban.