After 20 years, I have decided to update one of my reserve/guest snowmobiles. It is a 2003 SkiDoo MXZ Sport 600. It has the 1+1 seat that has served us well while the kids were growing up and when guests came to our cabin. I will be placing a spring order for a SkiDoo Grand Touring soon. The question I am putting out there is do I go with the 600 ACE or the 900 Ace? Im a fan of the 4 stroke as I'm riding a '21 Renegade with the 600 ACE. I know price difference is more than a thousand for the bigger motor but I am worried that the 600 isn't enough for a sled that will be primarily used for 2 people. Any advice or opinions out there would be greatly appreciated. Also, trade-in or sell outright the '03 MXZ thoughts. It has 4500 miles on it, never trailered, well maintained and garage kept. Thanks.