I've traded every year for the last 20 or so years, there may be a couple of times in all those years I kept a sled two years. The trade hit has always been high. I don't get rid of them because they won't make another year or two, it's because I ride a lot and if you think the first year hit is bad, look at what it is on a two year old sled with over 4,000 miles on it would be.
So in a way my decision is really based on resale, it's just I take the big hit up front and have a sled that's still marketable and not have to worry about having a sled I have to give away.
In my opinion one sled doesn't resale better over the other as long as you stay within the brand. When switching brands I seen it go both ways.