what is the best deal to go with, I'm going to buy an 18 tooth cog... it seems all the online retailers label all the cogs for single speed conversions, I see no conversion kits for fixed gear...

Can anyone tell me of a clear way to tell if I'm purchasing a "single speed/freewheel" conversion as opposed to a "fixed gear" conversion? I'd hate to buy a kit online & wait a week to find I bought the wrong conversion kit.