dude, please.... do a search or at least look through the newbies and techroom forums. There are numerous threads on this EXACT same topic, some from a matter of days ago.
what you have to watch out for is feednecks not fitting the Vlocity. We tried using one at Campaign when a halo went down, it wouldnt fit into any of our locking feednecks until we put tape around the loaders neck
If its between a Halo and Velocity then ide say a Velocity because its faster. You dont really need the extra speed on a Ion but in the future you will not have to worry about getting a hopper to accomidate the speed of a new gun.