BCY-X is a better material, especially for hunting or outdoor shooting in general. Stronger, more durable, less stretch/creep, very quiet. I like 18-20 strands for most any bow--that will be about the same diameter as 10 strands of Dynaflight '10.
Two big differences between X and 10.
First, X is made with SK90 Dyneema--the strongest available. '10 is made with SK78 Dyneema. For reference's sake, Dynaflight '97, 8125G, 452X, etc. are made with SK75 Dyneema.
Second, X has 17% Vectran in it. This further stabilizes the string, especially in higher temps, and seems to aid in making it quieter. 450+ and 452X also have Vectran, but higher percentages. With the better quality Dyneema in X, it doesn't need as much Vectran.
If you are ordering string material from BCY, you won't find Dynaflight '10 on the list anymore. It's now sold as "Force 10".
Chad