I'm no expert, but one thing that was a must for us was the Detroit Diesel Series 60. Better fuel economy, longevity, and performance. You can find them in coaches as old as 1994 (somebody correct me if I'm wrong.) But to get one with the 6 speed Allison transmission you need to move up to the 1995. I have heard a few first hand accounts of the DD60 and the Allison 5 speed tranny not working well together and causing some problems. So I guess I might prefer the 8V92 to the DD60 and Allison 5 speed. But the DD60 and Allison 6 speed is awesome!
Also, having purchased my coach only 8 months ago, and having looked at dozens and dozens of busses... I really noticed a difference between the converters. Sometimes on the outside, they looked the same, but the guts were quite different. After seeing all those coaches I decided for myself to go with a Marathon. And I'm glad that I did.
If you are still looking in 8 months, give me a call. I'm thinking I might want to trade for a coach with a queen bed and two bunk beds in it. We have another national tour scheduled for 2010-11 and our current situation won't be ideal. So if anybody knows of a Marathon (I'm hooked) with a queen and two bunks WITHOUT slides, let me know!
Good luck!
Adam