Simonis 860 is 90% wool and heavier then Velocity which is 85% wool (appx 21oz). The performance features are comparable in lab testing however Velocity is new to the market (Feb 07) while Simonis has been around for a long, long time. Installed on a table it is hard to tell the difference between the 2 because the finish and grain is almost identical.
Velocity is considerably less expensive but due to its newness it would be prudent to test it out on 1 or 2 tables first. I think 6 months from now we will all have a better read on it in terms of durability and real life performance. The initial response to Velocity has been very good but a this point in time we're not ready to push it as an alternative to Simonis - rather, we pitch it as an upgrade from woolen cloths such as Titan, Ultra and Forstmann.