(cheaper, but does not contain orthosilicic acid...is this necessary?)
Orthosilicic acid is produced by the reaction of water on the silica. Especially in the presence of acids. Being that the chemical analysis of DE includes about 10% moisture this also means that all DE will contain some orthosilicic acid naturally. So I would go with the Earthworks.
All zeolites are aluminum silicates. So there really is little difference. When I need zeolites for experiments I just go to the aquarium store and get a box of ammonia remover, which are natural zeolites. They are about $6 a pound.