Both parts of Gressoney, Trinité and St Jean, have access to the Monterosa Ski area. St Jean is a short drive away from the lifts, while you can ski to your door at Trinité. St Jean is the prettier and larger of the two, but Trinité has been described as "a pleasant, friendly little resort with a lot of character", while the "surrounding area is lovely, with great scenery and picturesque villages".
St Jean also has more restaurants, shops and accommodation. Both are good value. Generally, après-ski is quiet in the 2 resorts. This isn't party territory - you won't be able to buy cigarettes anywhere in Trinité, so you'll just have to try this whole “health” thing…
Gressoney has some of the best skiing in the Monterosa (200km of runs). Snow reliability is very good, thanks to the high altitude and machines. There is great terrain for intermediates, while better skiers have access to the excellent off-piste at Alagna. Beginners have lots of good slopes, but they are located all over the ski area and not particularly well-linked. There are minimal queues, due to excellent lifts and good links to Champoluc.
If you should happen to find yourself stuck up in the mountains, there are some great refuges where you can eat and sleep until the next day. Trinité and St Jean are great for accessing fun, varied terrain, while both have different strengths and weaknesses.