I have skied both and the comparison goes to the cost. Killington is cheaper except if you factor in the travel and exchange. Tremblant has become a world class resort and is priced as such. The experience is more than the skiing, it extends to the apres ski as well. There is no end of things to do and in that regard Killington cannot compare.
As to the night skiing, it will NEVER happen. This is a decision that is based on economics ! The skiing is to attract the skiers and the 4:00pm closure is to send them to the bars, restaurants and hotels. Lets remember that it costs a fortune to keep the hills open and lighted for the few who really want to extend their day. Intrawest earns a fortune off of every sale made in every business in the resort so it is in their interest to ensure that the skiing public frequent these establishments at the end of the day. It all comes down to money.
One additional point, for those who live close enough to frequent Tremblant on a regular or semi regular basis passes will reduce the cost and several options are available, I purchase the T-66 every year and enjoy a lot of skiing at a very reasonable price. Day passes can be bought online and I believe through Costco at a reduction as well, so options exist.